Hopefully Apple will improve OpenGL for gaming. It's dreadfully bad. I know gaming in MacOS was never optimal, but it was never as large as a 40%+ difference. It used to be just a few FPS. Actually, some games, at one time, performed better in MacOS than in Windows (WoW, Starcraft II, etc).
I've read somewhere that a user with an RX 580 did not notice any difference in FPS from Sierra to High Sierra. That sucks.
Nowadays, MacOS has more games than ever. The amount of Mac-compatible Steam games is pretty darn impressive, and there's lots of AA and AAA titles that are Mac supported. But without Apple putting any effort into OpenGL performance in the past 8 years makes it a pretty crappy proposition.
I don't mind on losing a couple FPS when it allows me not to have to restart into Windows. But when I'm litterly cutting my FPS in half, or when replacing a 7950 with an RX580 produces the same performance in MacOS, I tend to think something is wrong.