Thank you for the review! Yeah right now the "active" planet (e.g one recently dropped) runs itself at the full framerate, whereas inactive planets run at around 1/4 framerate. It's a lot of physics for the little fella to calculate and so for now, this is an unfortunate side effect of my optimisation.
I'll be continuing to work on optimising this and I'm hoping to bring the inactive balls up to 1/2 framerate sometime soon! Stopping the game from stuttering is my main task, as that feels really bad.