Physics Quality for specific objects.

I am Currently working on a level for my game that is going to be a large ball pit. I have a top layer of balls that are going to have physics for interactivity. I have made many optimizations getting it to 80fps. The thing that I need help with now is that I would like to set the physics delta time for only the balls. or any settings that would make the physics quality lower for better performance. Any help would be greatly appreciated! I have photos below to show what I am talking about.