Reason 1.Did you put a large box arround the map to fix any leaks? since if you do that VIS is going to have a very hard time because it thinks everything inside the box is the inside of the map.
It will render even the outside of the walls which under normal circumstances (without the box) would be excluded since they would be touching the void.
Solution:Remove the box and fix the leaks, ow and also another rule if you did this, ignore the following if you didn't use it

...
NEVAH USE A BOX TO FIX LEAKS!!!Reason 2.Maybe you have some cilinders or other very complex shaped brushes touching a wall, if so the compiler will brake it up and it will look very nasty (also ingame with lighting). This will also result in a longer compile time.
Solution:1. keep a gap of 1 unit (1 grid) between the wall, ceiling or floor and the complex brush so it won't touch it and be broken up into many pieces by the compiler
2. tie it to an entity (not recomended if its large or you don't have entities to spare)
These are 2 possible reasons for an one box/room map to have a very long compile time.
I'm compiling on a:
AMD barton 2500+ (which can run at 3800+, but never mind that
)
512mb mem
1.5gb virtual mem
Achio 32% at the moment, full compile takes about 15-20 minutes depending what the load of the pc is (idle or in use
)