glClipPlane so slow?

I have one simple model drawing with a display list at around 75 fps. When I enable my clipPlane it goes down to around 35 fps even when it is not clipping into it at all!

Anyone have any idea why the fps would drop so drastically?

P3 500, GEFORCE256

I have heard that clip planes use a texture unit but I am not sure. Either way I find them extremely slow as well so your not alone