There's a reason those QHD display phones are laggy; and it's not all Android itself. You need RAM and processing power to push around that many pixels.
I would take a smooth device ANY day over a few more pixels on a 5" screen rather than trying to cram a QHD screen in with 4gb RAM and need a 3000mah battery just to get a few hours of usage time.
Play with a Note5/Edge+ and then tell me it's laggy.
They absolutely fly and look stunning whilst they're at it.