I also had problems with xperia sp after updating to 4.3 - phone was slow to an extent of being unusable. However, even before update (on firmware 4.1) I found the phone quite slow.
I am very happy that I returned the phone to my carrier in exchange to nexus 5. When I got hold of it I really felt the change in responsivenes compared to sony. I cannot recommend it more - it has all the sony has + better responsivness, better screen, better battery life (yes, I have tested this), better camera minus bloatware. I got all of this for paying 50 eur more.
That was my second and last phone from sony. After having sony xperia S for 1 week and returning it because of wifi problems and after trying xperia sp and again returning it because of crappily written firmware I really wonder what are people at sony thinking about.
People, don't you see that sony does not give a **bleep** about customer satisfaction after getting their money? Of course, you can downgrade manually, but don't forget that this voids your warranty - staff at service centre would be more than glad to tell you you have just voided your warranty because you brought them phone that you had to downgrade because of crapilly written UPgrade!. In addition to that downgrade means spending your a couple of hours of your precious time correcting mistakes of multinational corporation that simply does not give a **bleep**. Throw your sony into trash instead, and use those hours to earn money with which you can buy new phone that will actually work.
