I have exactly same observation.
You would definitely need to go to "game battery saver" and turn on "optimization by game", then set each game to "low resolution & high frame rate". But this will only alleviate the problem, not fix it.
It seems to me that V20 thermal throttles a lot. Do an experiment - leave your phone for night, to let it cool properly, then run 3dmark benchmark. Then repeat benchmark a few times during busy day - you'll notice that the results are drastically different - hotter your phone is, the slower it is.
This really bothers me and I hope it can be fixed by a software update or rooting..