Poor SoT and little bit laggy

As in title. Running atm stock 5.1.1 with elementalx kernel ( tried going back to stock one ). At best get 5 h SoT but mostly 4 ( from what I can see people get on average 7-8 hours. Tried clean flashing it once again, tried lollipaloza ( meh ). In addition to that I think the performance of my N9 is pretty weak ( gets laggy on home screen, switching between apps, some games ). Any suggestion about fix ( maybe matter of ROM or kernel? )In worst case scenario will probably just send it back to HTC service.
Cheers