Primitive Collider slower then mesh collider

Hey,

I recently read that primitive colliders, such as box colliders, should be way faster then mesh colliders. So I decided to replace some of the mesh colliders in my scene with box colliders.

Part of the replacement where the colliders for the cars I use, which use UnityCar as driven physics.

So instead of having one optimized mesh collider for the cars I now had 6 Box colliders. As far as I read they should still be faster then the mesh collider. But strangely they ended up being way slower then before. Instead of an average of 10-12 ms spend of fixedUpdate, it now needs around 16 ms. (On iOS)

So this kind of confuses me as that is kind of the opposite of what I was told before.

Anybody has an Idea where the problem might come from?