You must have missed that almost all Android phones got hosed last year due to the Snapdragon 808/810. Also, im assuming your phone is up to date? My 5x is extremely smooth after I installed the march update.
And Pixel C owners and Nexus 5x owners
Edit: Ah screw it, opted in the 5x. Waiting for the ota now.
I swear I'm seeing lots of frame drops and it hangs a lot switching apps. I had a regular sized 6S just days ago, though, so that may be of importance.
I know about the 810, but expected better from what I read with the 6P. I totally notice it warming up even while doing mundane tasks.