This is the save of my fort where I encountered this problem when I was normally playing. I have cleared out all the tens of thousands of subterranean animal people and their corpses and items, and FPS did go from 0 to 1, but still, I should have the maximum of 100 FPS with this few stuff around.

I also noticed that after eliminating the animal people underground and reclaiming, their amounts still seem to double, as if the cataclysm never happened. I'm assuming there's some kind of "animalpeople quota", that keeps doubling and is fulfilled on every reclaim.