after upgrading my 2015 MBP to HighSierra i experienced awful slow game performance with 5-10fps.
Did some “cleaning” of the system and thought maybe internet was slow, but after a bit of google, i found a lot of entries concerning awful “ingame” performance of High Sierra. Not sure if there might also be a connection between games and the new file format. anyway, long story short i am back to around 30-50fps with a single reset: “RESEST OF NVRAM”
+++ Turn off your mac and turn it on while holding command + option + R + P, the screen should cycle on and off and you’re supposed to hold the keys down for 20 seconds. Afterwards, just let go and your mac will start up and the problem should be solved! +++
this workaround can be found on apple support site as well!
not sure its permanent, but so far it does the trick!