The reason shaders on level geometry is usually slow is because of the clipping the engine uses.

For example, a simple sphere model is visible, you are so close it covers the entire screen. The model will only draw the faces visible with the shader.

When the level covers all visible screen areas, it sometimes draws other parts of the level nearby in the background as well, so it's drawing a lot more.

This is why proper occlusion clipping is so wonderful, it clips out all those background planes because it knows the one visible is blocking it. BSP uses regions of levels for clipping, so theirs still extras. ABT is sort of a mix, and better, but not close.


NOt that this info will really help you, but it's some info which is always nice... i think.