How I Downgraded my Android Version to Make Things Faster

I had custom firmware (not made by Sony) for my phone, based on Jelly Bean (4.1.2). My 2011 phone is too old and too slow for this. So, I made the decision that I should roll back to Gingerbead (2.3.X), also custom, to preserve the rooting.

1. Download your custom firmware and Google Apps (GApps). 
2. From the firmware ZIP file, extract boot.img.
3. Download & Install Flashtool, which is a tool discussed in an earlier guide, specially for Sony/ Sony Ericsson devices.
4. Go to drivers folder in the same folder where you installed Flashtool (should be in C:\Flashtool\drivers). Click it and select your phone's model, fastboot mode and flash mode drivers.
5. Confirm that you want to install the drivers as many times as you need till the installation is finished.
6. Start your phone in fastboot mode by powering down, then holding down the menu key while plugging it in the USB port of your PC.
7. Open Flashtool, it should show you that it can recognize your device connected in fastboot mode. Click the button with lightning icon (top-left). Select fastboot mode.
8. Click select kernel to flash. Browse to the boot.img file that you extract from the custom ROM's ZIP file. Should be quick
9. Now, find a way to connect your SD card to your PC. Personally, I would wipe it clean of any files except music and photos. Copy the custom rom ZIP file & GApps file. Make sure you downloaded the correct GApps version.
10. Put the SD card back in the phone, disconnect it, power it on, when it shows you the custom firmware logo (in my case, Free XPERIA Project), press volume down button a few times. This should take you to recovery menu.
11. Wipe data, cache, then go to advanced > wipe dalvik cache.
12. Go back to main menu. Select install ZIP file from SD card. Now select your custom firmware ZIP file.
13. When it's finished, you can either reboot to see if that works, or you can just install GApps ZIP the same way and get done with it.
14. Reboot.
15. Everything should be okay. Mabrook!

The banner photo is from DriodViews. You can visit their guide which is specific for downgrading Samsung Galaxy phones here. Also many info was taken from the glorious XDA Developers.