I think that with collision, the pawn (or projectile or whatever) does an octree lookup to see what’s nearby. So although you might have thousands of those rocks on your map, only the ones closest to the pawn are relevant to collision.
Edit: Just to provide some additional info, I once had some meshes that somehow I screwed up the collision on. If you looked at the collision in mesh properties window, you would see the collision boxes stretching out infinitely. I could place those meshes in my game world, and they mostly seemed to work, but every time I selected one of them in the editor, the editor would hang for several seconds. You also couldn’t see the meshes in the orthographic views. From those observations, I think that the editor caches all the static meshes into the collision octree, so there’s an up-front calculation cost to save time on collision at run-time.