My last android update has slowed my phone to a snails pace and is now a little buggy. My phone is only compatible with the version from 2 years ago. Is it worth installing?
On my Xperia Ray the 11.0 builds feel slower than the 10.1 builds were. I've yet to try the latest 11.0 build where they switched from PMEM to ION.
There are plenty of different versions for you to try here.
http://forum.xda-developers.com/xperia-s/s-development However you need to unlock the bootloader to install any unofficial software.
There is an official way of doing it
http://unlockbootloader.sonymobile.com/ but this stops some Sony software from working (although you might not care). And some phones can be unlocked by using a test point to allow flashing of a hacked bootloader. Some phones can't have their bootloader unlocked & it can depend on who sold it to you.
The third option is to run cyanogenmod on top of the Sony supplied kernel
http://www.xda-developers.com/android/locked-bootloader-xperia-s-and-want-cm10-no-problem/ cyanogenmod is just the front end and not the Linux kernel and drivers, but some features need new drivers. So you're likely to have issues moving forward, but it could make it bearable in the short term.