"Vis" MDLs are really processed by the renderer, so apply some level design rules for the sake of your frame rate.

In the manual under "Frame Rate Optimization" you'll find many hints about good and bad level design. A MDL is put into the render chain when even a tiny part of it or a tiny bit of its bounding box is visible. So a "level of 18 large MDLs" is not a very good idea. If you really must use MDLs, take at least care that they are static for reducing their bounding boxes, and use many smaller objects instead of 18 large objects.