If you press P in-game, you can delete all pedestrians and cars and prevent more from spawning. That should help if you're CPU-bound.
Viewing post in The Unbreakable Gumball comments
thanks for that I think it may be helping but I have also noticed that sometimes after playing it for a while it slows down, like when I first open it the game runs better than it does after playing for a while not sure why (after restarting the game I think I have determined the cause. When booting the game I go straight into free roam then after swinging around a bit I went into the menu to equip a different suit and after exiting the menu it was running significantly worse than before)