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Check out this FAQ page: http://talk.sonymobile.com/t5/FAQ/Does-your-phone-feel-slow-and-laggy/m-p/347824#U347824
There seems to be a software problem. First backup all data, then reinstall the phone software through PC companion or SUS. If you still have problems, then take it to Sony repair centre.
Keep it pressed for more than 5 seconds,simoultanously.
press up volume key+porwe button...it really works.
I'm also facing the same problem. please help 😞