Hello I have the same issue, and please don't tell me it's a software issue because I erased everything system/dalvik/cache/data , the only thing left is the recovery and even the recovery is slow. And this is happening following one of your reparation because the phone was unable to turn on again. The phone was keep freezing since that date but was still working, and now it's very very slow I can't even unlock it because it take so long to just display the unlock code that it come back to sleep before I can even see the unlock screen.
So I guess when your support repair the phone and changed the motherboard (that's what they did) they broke the processor or if the processor is directly integrated to the motherboard then it is dead, because it's clearly that kind of issue. I work in IT I am software engineer so I know what's a software issue is, and it is an hardware issue...
So the quality of your device suck.
I guess as the waranty just ended like 1 month ago you will not even treat my problem....