i bought arkham knight for cheap on a lark ($5 with season pass). i had tried it in november 2015 (after it was relaunched) but back then i had a 280x, only 8gb ram, and was playing in 1080p. i thought performance was crap so i refunded the game through steam.
setup:
2560x1440
1070
4690k 4.5ghz OC
16GB ram
game on sandisk ultra ii ssd
i started with everything to high, AA on, all gameworks on. with these settings, the start of the game through finding poison ivy (like the first 15 min) was fine, i was getting 70-90fps except when walking over the AR challenge start points which for some reason drops fps to the teens for me. i played for a couple hours through going to scarecrow's toxin facility, and then the fps turned to shit, i'm talking like 20-30 fps during batmobile, 40-50 fps on foot. ok so i restarted the game, maybe it was a memory leak or something. nope, restarting the game it was back to 60-90 fps again but once i used the batmobile it would tank the fps everywhere, including even if i returned to foot.
i messed around with the ingame settings, turned off the ingame fps cap and used RTSS to cap at 60 and i think things are ok now, but we'll see after a few more hours of play.
games like this are probably one of the things that people cite when they say PC gaming is too much of a pain to get into. luckily bad ports like this are rare and not the norm.