If it is taking 10 minutes for a small level to compile then you might have Preview unchecked. Check it and see what happens.

I have a fairly detailed 6000 portal city level and it takes around 2 minutes with it checked. Unchecked takes 5-6 hours. Pentium 4 2.4GHz with 512RAM - A6

It's shaded with high quality lighting(using the new map compiler, there's actually not a preview box, only 2 sliders which reads visibility on/off and lighting high/low).