This (http://www.student.ru.nl/rvanhoorn/optimization.php?chapter=func_detail) is essential reading for anyone new to mapping. Don't leave it until your map is half finished with compile times of days before thinking about optimizing your map.
By tying a dozen pillars (world brushes) on my map to entities (Ctrl-t) with type func_detail, I reduced the vvis part of the compile from over 18 hours (I always stopped them before they finished) to 1 hour 23 minutes. :D
I still have lots to do and hope to bring this time down further but it would have been a lot easier if I had read the linked article earlier.
Good find. I have cut my compile time down from 11 hours to 40 minutes using this and "nodraw" textures combined with lower skyboxes.
I find this is important, it's amazing how many maps you see miss out on this. Fair enough it takes longer to optimise it like this, but the results are smoother maps with small file sizes and faster compile times.