I've been working with modular ships in a shmup, and am having substantial problems. I can have about ~20 modular ships (via many child objects for each ship) active and on screen, each with ~30 total objects, without performance problems. If I increase the object count to ~40 per ship, without adding any additional scripts at all, I get a slideshow.
Everything points to the pure object count. Nothing is being instantiated at the time, and none of my scripts search through the whole object database. The game uses only ~25% of my available memory during all of this. My GPU barely even notices the load from the simple meshes being used. The additional objects do add more box colliders (about two box colliders per three objects, on average), but no additional rigidbodies nor any additional scripts. Collision detection on each rigidbody is set to discrete. The game runs at 120 fps (including physics) and is smooth as butter otherwise.
What can I do to improve the situation, or at least get a better idea of what is going wrong?