I have lumia 640,the games on windows phone 8.1 works fine,smooth.I uptadet to last version PR,i make hard reset.Games work hard,not smooth are limited to max 20 fps.Is a bug?How i can send this feedback to Microsoft?
try running the game while having battery saver and airplane mode turned on so we can discard the usual unusually heavy background activity thats become standard in w10
if it still plays at 20fps, try reinstalling the game, then if that fails you can try a soft reset, and then a hard reset, final solution is to go back to 8.1