I had a similar problem before, but it is more pronounced in this game. Basically if I run the game on android device without ads it seems to run fine and smooth. when I put ads in (admob), it starts to stutter after the ad plays in the gameplay.
It seems to be when sound effects are playing for the first time after the ads play. I have a very simple sound system - I load all my effects at the start of the game and then just play them. Its as if all the audio is being disposed of and has to reload again after an ad plays (which is not what is happening in the code).
Any idea what may be causing this? how to fix?