Cause this game like many other dosnet realy support multi monitor setups i get from time to time some res bugs. Even thought the res was 1080p on bothe monitors and the game ist set to 1080p too, after switching screens the game sometimes scrambles the res to something else. Since a while you cant fix that ingame (at least in ranked) anymore and so i am forced to play with wrong res like sometimes even 480p. Its unplayable tis way cause you cant see ■■■■ like that 
I think its cause of the whole benchmark thing and stable play but lets get real … the performance is only that crappy cause its bad programmed and the benchmark is just unnecessary crap too. If the game laggs thats it and just wait till devs get the game stable if you cant handle lag.