Physics.Simulate time increases exponentially relative to rigidbody count

With 90 rigidbodies, Physics.Simulate takes 6 milliseconds to run each frame.

With 180 rigidbodies (two of the above groups, separated), Physics.Simulate takes 228.

If the PhysX system is optimized to only check for collisions between objects when they are close, (as is not the case between the two 90-rigidbody-groups), why does adding another 90-rigidbody set somewhere else increase the Physics.Simulate time exponentially?

Have you tried setting a Maximum Allowed Timestep in the Time Manager? Unity - Manual: Time