For flat mesh, is there a performance difference between box vs mesh colliders?

big difference? little? thanks, I appreciate any help.

Primitive colliders are easier to calculate than MeshColliders. So yes, there is a difference.

MeshColliders are fine for static meshes. But for dynamic objects / rigidbodies you should use Primitive colliders or compound colliders made up of primitive colliders.

If you need Rigidbodies with MeshColliders you should make them convex in order to collide with other meshcolliders. However MeshColliders don’t work with flat geometry because a RB collider always needs to have a volume. A flat mesh like a plane doesn’t have a volume.