I play on Android and do not normally have a problem with this. However, if I swap to another app (browser, text, call, etc.) with the game running and then swap back to it, that's when the frame rate issues start. I've noticed this in a lot of other games, as well. It's like the phone puts it to sleep, then it doesn't start up again properly when awoken. I'm not sure if the issue is on your end, but that's just in my experience.