Exactly same situation as me
I upgraded to ICS thinking all the Gingerbread problems on the Xperia U would now be solved, however as you've mentioned the ICS just kills the Xperia U's performance, gets so bad at times you will press an applications button and it can take up to 5-10 seconds to open. (Sometimes the phone will hang and require a hard reboot).
You cant revert it to Gingerbread, but you will need to send it to Sony to do it, they did it twice for me when I experienced the slow ICS performance (the reason I sent it back to them a 2nd time is I thought that maybe my Gingerbread was corrupted to begin with which caused the inital ICS update to go bad - but alas the 3nd time round with a fresh Gingerbread install, then udpated to ICS and still the same ridiculous unusable Performance responsiveness.. Grrr frustrating.
So now I'm stuck on gingerbread on a Phone that when I bought was promised that it was upgradable to ICS in a COUPLE MONTHS.
Gingerbread has its own problems:
-Phone will also intermittantly restart itself on occasions of lagging for whatever reason.
-Simple apps like BBC iplayer, Facebook or twitter will randomly 'Force close' and fail to open, you have to try a 2nd & sometimes even 3rd time
-Still cannot receive an incoming call whilst listening to music, the music dies, the calls answers, but cant hear anyone (And sone have known about this problem since August 2012)
-Dunno if anyone else also has the problem with their music distorting and glitching randomly (but still at least 80% of the time whilst you are multi-tasking .i.e Listening to music whilst simply navigating a Twitter feed or Facebook or checking an email.
As I say. its been to them many a times and it just comes back wit the same problems.
Im busy fighting a principle case with them (Or rather my Provider to refund my money back under these circumstances), that even though I've had it 4 months, it was due to them telling me that updates where being released to resolve all these problems - but nothing is fixing anything they release - bunch of clowns in their support development team - . I bet the phones hardware is poor and they are trying their best to resolve problems by using the Software to cater for teh flaws, but its just not working out, . .. hence the duration of these problems still persisting & also the fact that many providers didnt even release the ICS updates to their users as its just an even more memory hungry horse which the sub-spec Xperia U cant support!
So yea, either call the network provider you bought it from or contact sony chat support direct and they will book you a job to have it Repaired (i.e. reverted back to Gingerbread, its got problems, but at least you can still navigate your phone).
Good Luck!!
