In many cases, objects that are too small to be discernible on-screen (as a function of their size and distance to the viewer) are not being rendered. Some cases still need to be optimized further.
That said, rendering is not the most costly position on the budget list. The sum total of updating the multiple simulation-relevant subsystems that a body is subject to (physics simulation, collision detection and response, composition, classification and evolution, and lots more) takes more time for most systems.
We're constantly working on improving performance, and suggestions are always welcome.
Cheers,
George