The building of the BSP tree doesn't take that much time, most of the time is spent using that data to calculate visibility and to create shadow maps (which is why preview and flat shaded levels build so much faster).

We are looking at ways to scrap the BSP since, as some of you have discovered, with newer hardware and certain level types (i.e. large outdoor areas) the improvement it gives isn't as great as it use to be. But, in many cases, you will still see a major improvement in both visibility and collision detection.

Like most things, it is a trade off. We could make all the lights dynamic but you would be restricted to 8 per level (and it would only work well on fast hardware). We can remove build times, but collision detection and visibility calculations would take much longer.

Right now the best advice I can give is to build your levels in preview mode (the City Level takes ~90 seconds to build on a fast machine in preview mode).