There are several things you can do to improve your phone’s performance.

Check the following:
Available RAM and phone memory – Begin by checking how much available RAM and phone storage you have. If these memories drops low the performance may be affected. We recommend that you have at least 10% free phone memory. You can check your phone memory by going to: Settings > Storage.
You can free up some phone memory by moving supported apps to SD card or internal storage.
You can check your available RAM memory by going to: Settings > Apps > Running. Here you can see all applications that are running in the background of the phone. Closing apps here will increase your available RAM but note that most preinstalled applications are programmed to be running in the background, you can close them but they will soon be started again. But check if you might have any third party applications running in the background that drains your RAM memory.

Disable apps – This feature is available in Android 4.0 (ICS) and later versions. You can disable both third party and most preinstalled apps so they don’t run in the background. You can do this by going to: Settings > Apps > All > Select your app > Disable.

Live Wallpaper – Do you use any Live Wallpaper? These generally consume a lot of RAM memory making the phone feel slow and sluggish depending on the available RAM of course. Try to use an ordinary wallpaper if you want to improve performance.

Widgets – Having many widgets active on the home screen will also use a lot of RAM memory. So reducing the amount of widgets on home screen will speed up the UI performance.

Up time – When was the last time you rebooted your phone? You can check this by going to: Settings > About phone > Status > Up time. It doesn’t hurt to reboot your phone every now and then.

Task killers – We don’t really recommend you using task killers if you’re not sure on how to configure them. By default most task killers are set to close all applications running in the background which can cause problems with applications, widgets and also drain battery since many preinstalled applications will automatically start running again.

If you have an Xperia phone with 512MB of RAM memory and plan to update to Android 4.0 (ICS) you should be aware of that Android 4.0’s hardware requirements are more advanced than previous software’s so your phone’s performance may be affected.

You can find more information about that here: http://developer.sonymobile.com/2012/03/30/learn-about-the-technical-differences-between-gingerbread-and-ics/

Solution

On Xperias with removable battery
Note the battery % . Take the battery out of your phone and re-insert it after 5 minutes or so. Switch it on and observe the battery %. See if there's any significant drop in battery % ( 5+ % or more ). Also there should be produced a gap in the battery graph, like this Battery Screenshot. Most probably this problem has been corrected.

On Xperias with built-in/embedded battery
Note the battery %. Force shutdown your phone by following this key combination: 

Press and hold the "Power Button" and "Volume Up" for 10 seconds until your Xperia vibrates 3 times. Now switch it on and observe the battery %. See if there's any significant drop in % ( 5+ % or more ). Also there should be produced a gap in the battery graph like this Battery Screenshot. Then most probably the problem has been corrected.

How to avoid this error in future:
This error is usually produced when the phone is left connected to the charger even after battery becomes full. So disconnect the charger as soon as the battery charging status becomes full [ ( Settings > Battery > Full ) or (Settings > About phone > Status > Full) ]

To help deal with it, install any app from google play that fires up any kind of alarm or notification whenever battery goes full.

To use Android phones efficiently, users should be aware of the different types of phone memory. This knowledge is important in order to understand, for example, where music, photos and videos are saved; how many apps can be downloaded from Android Market; and how photos can be copied to a PC.

Generally, all Android phones share the same basic memory setup. What differs is how much memory is available to you via the different types of memory, and whether your phone uses an external SD card or an internal memory chip. Any information specific to the particular phone model described in this White Paper is noted as such.

Please note that when internal memory is used, the figures you see in the phone information menus may appear to not match with the total amount of stated physical memory. In other words, the figures might not seem to add up. The reason for this is that some sections of the memory may use two memory cells instead of one for every storage unit, in order to secure storage integrity. The need for such “double storage” depends on the type of memory chips used and may therefore differ between products.

The types of memory described below are consistent with the terminology used in Sony mobile phone menus and in other content relating to 2012 Xperia™ phones:

1. Dynamic Memory (also known as RAM, or non-persistent memory, because everything in RAM disappears when the power is turned off) is used as “working memory” when the device is actually running, and is shared between the operating system and all active applications and services. Therefore, the amount of Dynamic Memory influences how many applications and operating system services can run at the same time. In Android™ phones, the operating system automatically closes applications and services that are not being used. However, such automatic functionality has limits. For example, if a lower amount of RAM is assigned to a certain release of the operating system, phone speed will be impacted.

If you experience problems with RAM, for example, if the phone runs slower than usual or if the Home application restarts frequently when you leave an application, you should minimize the use of apps that run all the time. Such apps could include, for example, applications that frequently download social service updates. You could also consider using a static wallpaper instead of a live wallpaper.

To see which apps and services are currently active, go to Settings > Applications > Running Services. You should have at least 50 MB, and ideally 100 MB or more, of free RAM to avoid slowdowns and application restarts.

You should also be aware that if you update the phone to a later Android release, the load on the built-in Dynamic Memory will increase due to the addition of more features. As a result, the phone may run slower after an update.

All the memory types described below (in sections 2 to 5) together comprise “persistent” memory. What this means is that all data and content stored on these sections of memory will “persist” after the power is turned off (in contrast to the non-persistent RAM). Persistent memory can therefore be used for storing applications, images, music and any other content which can only disappear after being explicitly deleted.

2. System Memory (also known as “System partition” or “system”) is used for the Android OS and for most applications that are pre-loaded from the factory. This type of memory is normally locked, and can only be changed through a firmware upgrade. There is usually some free space available in this section of memory. However, since it is locked, you cannot save apps, photos or any other content to this memory. System Memory is reserved for future firmware upgrades, which almost always need more memory than the original firmware. You cannot see or influence the use of this memory.

3. Phone Memory (also known as “Data partition” or “data”) is memory type that is used as working memory. It can be compared to the C: drive on a PC or to the startup disk on a Mac. All applications downloaded from Google Play™ or other sources are installed (at least initially) to this type of memory. Some can later be moved to another memory.

In this type of memory, as with System Memory, all applications have an allocated area which no other applications can access and to where the applications can and usually do save their data (such as phonebook, calendar, notes, and e-mail applications).

Phone Memory will tend to fill up as a result of normal use, the use of applications saving their data, and you downloading and installing new applications. Therefore, the larger this memory is from the start, the more applications you can download and use.

If the Phone Memory starts to get full, the phone slows down, and in some cases it might no longer be possible to install more app's. You should always ensure that you have at least 50 MB of free Phone Memory. If not, you should consider removing some apps that you seldom use, or move some applications from the Phone Memory.

You can see how much Phone Memory is free under Settings > Storage > Phone memory. You can also view Phone Memory availability and usage information under Settings > Applications > Manage Applications.

4. Internal Storage/SD card (also known as “sdcard”) is the memory used for:

• Content such as photos, movies and music which is added, for example, as a result of the user taking photos with the camera, downloading media files, and performing file transfers.

• Certain applications to store data in cases where larger amounts of content are involved. For example, applications for games and maps need to store larger files which would not fit in the Phone Memory.

• Applications that can be moved after installation from the Phone Memory. Note that not all applications can be moved, and in such cases the option to move the particular application will not be available. Typically, apps running as services, apps with widgets, or app's for live wallpapers cannot be moved. Also note that when apps are moved to the Internal Storage or to the SD card memory, a small part of the app will still remain in the Phone Memory.

This type of memory differs most between different Android phone models. In some models, a large amount of internal memory is built into the phone and is referred to in the user interface as “SD card” memory. In other cases, the phone features a memory card slot and a removable memory card that is bundled with the phone. No Android phone can be shipped without this memory type whether it comes as built-in storage or in the form of a removable memory card. The advantage of having an external memory card slot is that a user can replace the memory card with a larger one later on. In contrast, built-in internal memory cannot be extended. The drawback for the manufacturer is that a removable card is more expensive. Therefore, at a certain price level, a manufacturer can offer a larger amount of memory if it is built in, everything else being equal.

You can see how much Internal Storage is available under Settings > Storage > Phone memory.

In some Xperia phones, the three areas of persistent memory (System Memory, Phone Memory, and Internal Storage), together with some small memory allocations for system operations, share an amount of built-in eMMC memory.

Note that in some products you may find both a large internal memory and a memory card reader slot. However, on the current Android platform, the card reader slot does not work in the same manner in a phone with large internal memory, for example, a phone with only a memory card slot. Generally, while you can access content (such as videos, photos and music) on this optional memory card, you cannot in general save anything to the card. Some applications, for example, a backup service application, may still be allowed to do so. In effect, this means that some products feature a fourth type of persistent memory, called “External Card”:

5.  External Card (also known as “ext-card”) is the name for the removable SD memory card in products where there is also Internal Memory and where this Internal Memory is referred to in the phone’s user interface as the “sdcard” memory. This External Card memory can generally not be written to from the phone, but it can be used (by the user) to store content from other sources. For example, you can write to this memory from a PC when the phone is connected to a PC and when the External Card is mounted. Some applications on the phone may in some cases, however, also have permissions to write to the External Card.

Generally, you should not save photos, videos and other personal content solely on the internal memory of a phone. If something should happen with the hardware, or if the phone is lost or stolen, the data stored on the phone’s internal memory is gone forever.

In a phone where an SD card reader is the main memory, it is relatively easy to take the card out and copy all content to a PC or Mac, or to an entertainment device with a memory card slot. In a product featuring Internal Storage as the main memory, it is not possible to physically remove the memory. Instead, any critical or high-value content must either be transferred over a network (mobile or Wi-Fi) or via a cable. To facilitate the transfer of data via a cable, most Xperia phones supports the Microsoft standard, Media Transfer Protocol (MTP), which makes it possible to easily transfer content back and forth between your phone and a PC. For Apple Mac computers, a special application is available with built-in support for MTP. This application can be downloaded from the Xperia Support page.

Note that you do not need to back up or make a copy of applications that you downloaded from Android Market/Google Play™. They can normally be downloaded again if you have set up a Google account to work in your phone. You can find the apps which you have purchased under “My apps” in Android Market/ Google Play™, so you will not need to either pay for or search for them again.

You can find more model specific information about memory in  the dedicated White Paper for your phone model.

http://developer.sonymobile.com/downloads/group-whitepapers/

If you're having problems connecting your phone via USB there is a couple of things worth trying.

If your phone is not detected at all :
First make sure that it's not a problem with the cable. If possible try using another USB cable.
You can also try to connect the cable to another USB port on your computer. Don't use any USB hubs.

If the phone still won't get detected there's one more thing you can try from the hardware side. Connect your phone to another computer to verify if the problem is located on the phone or computer.

If your phone gets partly connected

Most Xperia phones have two USB connection modes. MTP (Media Transfer Protocol) and MSC (Mass Storage Device Class). You can switch between these modes here; Settings > Xperia > Connectivity.

In MSC mode ordinary mass storage drivers will be used, so the MSC mode works on most systems. Mac, Linux or PC. If you experience problems in MSC mode please check that a SD card is inserted in the SD card slot in the phone.If you still can't access your SD card on your computer you should check your device manager and that proper mass storage drivers have been applied. You can access the device manager by right clinking on "My computer" or "Computer" depending on Win OS version.

If you can't get MTP connection you should first make sure that PC Companion and/or Windows Media Player (Latest version, Win XP only) is installed on your PC.

If your Xperia phone is running on Android Gingerbread you can try to clear data for the application called MTPapp. Found here in your phone; Settings > Applications > Manager applications > All > MTPapp > Clear data. You can also try to enable or disable the USB debug option found under Development settings in the phone.

If your phone gets detected as another device such as PTP instead of MTP, try the following. (Win XP only)
1. Install the WMF redistributable from http://dl-desktop-apps.sonymobile.com/stage/WMFDist/WMFDist11-WindowsXP-X86-ENU.exe
2. When installed, restart the computer.
3. Plug the phone and see if PCC can detect it.
4. If not, check in windows device manager if the interface is still PTP. If it is PTP, then:
  a. Select to update the driver.
  b. Select to install manually.
  c. Select to manually choose the type/compatible device and from the list select “Portable device”.
5. If still doesn't work, in windows device manager, select the PTP interface, select to un install it.
6. Once it is un installed, unplug your phone and re-plug it.

Problem getting connection for update

First make sure that you're phone is turned off and that you press and hold the specified key for your phone model while connecting the USB cable (usually volume down). The notification LED should turn and stay green. If it turns blue, you're pressing the wrong key, check instructions in Update Service (SUS) or PC Companion (PCC).

If the notification LED goes off or if the phone boots into charging mode it's probably a problem with drivers. In this state leave the phone connected to the computer and open the device manager (see instructions above). Look for any unknown or not properly installed device (usually marked with ! or ?). Right click on it and chose to install the drivers manually from a specified location. Point the search path to the drivers folder in PCC. Now disconnect your phone and reboot your computer and try again. If it still fails try to uninstall the device via device manager, disconnect your phone, reboot your computer and move your USB cable to another USB port and try again.

Also, if you are experiencing any problems running an update via SUS or PCC make sure that you're logged in as a local admin on your computer and that firewalls, anti-virus and other sync/COM-port related programs are disabled/uninstalled temporarily.

Here are a few tips on settings you can change to capture better pictures in low light conditions with your Xperia phone.
Use Normal capturing mode instead of Scene recognition capturing mode. Once you are in Normal capturing mode, use the Party scene setting and/or the Multi autofocus setting for better results.
Use Fill flash mode.
Use the Infinity focus setting when recording videos.
Use the camera key correctly. Press the camera key halfway down to activate the auto focus setting. Press the camera key fully down to take a photo when the focus is locked correctly.
Your subject should be as still as possible while you are taking a photo.
Your camera should remain as steady as possible.

To use Normal capturing mode
Activate the camera.
Hold the phone in landscape orientation.
Drag the slider button at the top right of the screen accordingly so that the still camera is selected.
Tap the capturing mode icon at the top left of the screen, then select Normal.

To use the Party scene setting
Make sure the camera is in Normal capturing mode.
Press the Menu key, then select Scenes.
Tap Party.

To use the Multi auto focus setting
Make sure the camera is in Normal capturing mode. Press the Menu key, then select Focus mode. Tap Multi auto focus.

To use Fill flash mode
Activate the camera.
Hold the phone in landscape orientation.
Drag the slider button at the top right of the screen accordingly so that the still camera is selected.
Press the Menu key, then select Flash.
Tap Fill flash.

To use the Infinity focus setting
Activate the camera.
Hold the phone in landscape orientation.
Drag the slider button at the top right of the screen accordingly so that the video camera is selected.
Press the Menu key, then select Focus mode.
Tap Infinity.

The Xperia models come with different versions of Android so the options for making a backup looks different depending on version of OS.
Check your Android version in this menu.
Menu > Settings > About Phone > Android version.

For Android 1.6 (Donut) phones:
First go to this menu to enable Unknown sources: Menu > Applications > Check Unknown sources. Now open your browser and go to the address: m.sonyericsson.com/backup. A download of Backup and restore will appear in the notification bar, drop it down and when it’s finished downloading press on it, select to install. Now Backup and restore will appear in your application drawer. Open your application drawer and press Backup and restore to make a backup of your phone data to the SD card.

If you’re going to update your Android 1.6 phone to 2.3 or later skip the step above and read the information below.

For Android (Éclair) 2.1 phones:
Same as above, difference being that the Backup and restore application is included in Android 2.1. But if you’re going to update your Android 2.1 phone to 2.3 or later you should read the information below.

For Android (Gingerbread) 2.3 phones:
There is unfortunately no backup application included. Read information below for more info on how to backup your Gingerbread phone.

Note that if you’re updating your phone from Android 1.6 or 2.1 to 2.3 or higher this version of the Backup and restore application will no longer be supported. So therefore I suggest that you use any other application from Market to backup your phone data.

Like MyBackup, which is available in both free and paid version. https://market.android.com/details?id=com.rerware.android.MyBackup&feature=search_result#?t=W251bGwsMSwxLDEsImNvbS5yZXJ3YXJlLmFuZHJvaWQuTXlCYWNrdXAiXQ.. The free version is limited to 30 days and can't move backups between phone models.

If you’ve already made that update from Android 1.6/2.1 to 2.3 check this thread for more info:
Lost Backup with Android Gingerbread upgrade http://talk.sonymobile.com/thread/19718?start=0&tstart=0

This is no official solution.
Some phone models like Xperia S, Xperia P and Xperia arco S have a backup application installed. This application can backup your phone data directly to Internal storage or SD card (depending on model). Check your application drawer for Backup & restore. Later Xperia phones can also use PC Companion to make backups and store them on your computer. You can download PC Companion from our website. http://www.sonymobile.com/gb/tools/pc-companion/

First of all make sure that your phone is running on the latest version of the firmware. You can use PC Companion to check for software updates for your phone.

http://www.sonymobile.com/gb/tools/pc-companion/

Having the latest version of the software and still got problems? Then you could try to boot your phone into safe mode to check if the reboots might be caused by some application in the phone.

To boot your phone into safe mode looks a bit different depending on model, but generally you power on your phone and when the text Sony Ericsson or Sony appear you press and hold the menu key. You should see the text Safe mode in the bottom of the screen when it has started. *For phones using soft keys you tap repeatedly on the menu key when you see the Sony logo till the phone boots up.

Still having problems? Please feel free create a thread or post in an existing matching thread here on the forum.

Here are a couple of tips that will increase your battery time.
First of all make sure that your phone is running on the latest version of the firmware. You can use PC Companion to check for software updates for your phone. http://www.sonyericsson.com/pccompanionhttp://www.sonymobile.com/gb/tools/pc-companion/
You can also check battery consumption via the menu; Settings > About phone > Battery use. Or in later Android versions; Settings > Battery.

Features that can be disabled or reduced when you don't need them: 
Dim the screen.
Turn off 3G.
Turn off data traffic.
Turn off Bluetooth.
Turn off Wi-Fi.

By doing this you will increase your battery time. But if you're still experiencing a low battery time you can try the following as proposed by many users on the forum.

When phone reaches 50% in battery level turn it off and pick out the battery for a few minutes. Put it back and connect the charger, now charge the phone turned off for about 5 hours or overnight. Start the phone as normal and check if the battery life time has improved.

Later Xperia phone models also offer the feature Extended standby mode which can be found here: Settings > Battery.

STAMINA mode will constantly re-evaluate your usage and update the expected standby time accordingly. This standby time will completely vary depending on your own personal usage. You may see that when you first start using your handset, the Standby time will be higher than what is expected after you have used the handset for a few days. This is due to STAMINA mode having no previous usage history to estimate from. If you have a very high usage level, you may only see a slight improvement between having STAMINA enabled and disabled. STAMINA mode is more effective the more often the display is turned off as this is when the power saving features kick in. If the display is rarely turned off, you may not see much improvement in the battery performance.

There’s a lot to Wi-Fi parameters, settings and such that can cause problem for you to connect or get Internet access via your Wi-Fi connection. Here are a couple of things worth checking if you're having troubles with your Wi-Fi connection.

First of all, begin with reboot both your phone and router.

If connection problems still persist, make sure that both the phone and the router is running on the latest version of the firmware. You can use PC Companion to check for software updates for your phone.

http://www.sonymobile.com/gb/tools/pc-companion/

If you feel insecure on how to update your router firmware, contact your router manufacturer.

Still having connection problems?

Check these router settings:

Network mode/ speed: Try setting this to auto or mixed mode instead of b,g or n.
SSID and password: Make sure that none of these contains any special or extended characters.
DHCP: Should be turned on. You could also try setting a static IP if you're having troubles getting Internet access via your Wi-Fi connection.
Mac-Filter: Should be turned off, or make sure that your phone is set as allowed.
Channel: Try using another channel, preferable 11 or lower. Note that due to legal reasons channel 12,13 and 14 is not available in all countries. If you're using the phone without a SIM-card or you don't have any network reception channels 12-14 is also disabled and 5GHz is not available.
By using the application Wifi Analyzer you can check what channel that would be best for you to use.
https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer
Internet: If you get a connection but you are unable to access internet, test if the connection works by opening the routers settings page from the phones browser. This is usually done by entering the IP-address to the router (http://192.168.0.1, http://192.168.1.1 or similar). If you can access this page the connection between the phone and router works and you will need to check the internet settings in the router.
If you feel insecure on how to change the parameters above check your router support documents or contact your router manufacturer.
If none of the tips above would help, you should try to connect to another network to see if it works.