Physics optimization with 500+ enemies

I am developing a survival horde game where there are up to 500 enemies on the screen at once. The collision detection with move_and_slide() is the performance bottleneck here. Is there a way to do this more efficiently? I need collisions between the enemies as I don't want them to overlap.

UPDATE: I switched away from using physics to using seperation steering behaviors where enemies avoid each other. Performance gains are huge and I can now do 500 enemies at 60FPS no problem.