Level blocks render more slowly than model polygons. If you had a 12-poly model of a cube, it'll render faster than the block of the same size and stuff. In my experiment, I had everything in the view. I used nothing but models. I had one block to prevent the "no geometry" error, but it was a fairly small block.

Edit: I forgot to mention that the exception to this, which makes level geometry run faster than models, is when you have a bunch of models on the view at once.