While playing your test level:
Hit the "[" symbol on your keyboard for wireframe mode
Hit the "]" symbol on your keyboard for shaded/textured mode
If you hit your "TAB" key, a status box will pop up and give you a run down of what the engine is processing. This is an EXCELLENT to find bugs. In the bottom portion of this is a small menu that allows you to toggle settings in the game (lights, collision, display mode, entities, portals, etc). I have been able to solve so many issues with "speed problems" and "crashing" using this status box.
-This...is my boomstick!