I'm not really sure a lot of people have Marshmallow on their devices but I really urge you to upgrade. The system takes up very little space and RAM, so the gameplay is as smooth as possible.
Isn't it still in Dev preview? I don't think there's any real ROMs for it, apart from Nexus devices. Lollipop is a little clunky on my N7 2012, has some lag I can't get rid of.
Android 7 and above is currently the best for gaming. But android 10 and above is just perfect. In addition, make sure it has a bigger internal memory for the smooth running of a game.