I am using HTC 820 Desire and I bought it on January 4th, 2015. The phone was working fine till now until for the past few days it started giving me some problems. 1) The phone is getting heated up very quick when I am on WiFi/2G. 2) My phone was updated to Anroid 5.1, after which the phone became dead slow. Please advise.
I believe you have a lot of apps running in the background. I've experienced the same problem, what I do is stop my apps then I found this very useful app named Force-Stop It! (link). It stops all the running apps. Keep in mind that messengers and social networking apps will stop working after this so I suggest to open them later so you won't miss messages and other notifications.