Is it true: The less polygons a mesh has, the less a mesh collider of this mesh affects performance?

This question sounds like a tongue twister.

It would be nice to know how much of a burden mesh colliders really are, because I'm making a 3d game where rooms and corridors are big meshes, but they rarely have more than 30 polygons. I could use primitive colliders, but that would mean there would be a primitive collider for every angle of walls and a few for the floor (because there are no primitive triangular colliders).

So basically what's the approximate performance drop comparison between primitive colliders and mesh colliders? Is it something along the lines of 10 box colliders = mesh collider with 30 polygons, or maybe it's 100 box collliders = mesh collider.

I don't care about the exact values, I'm just trying to describe the question as best as I can.