Hmm... I don't know if I've suggested this before, but here goes.
72 on a 3.2 ghz computer with no effects or enemies is still a bit low. Everything you add, every particle effect and NPC will add to your system load.
Have you subdivided the level into object sections? The best way to do it is to divide the level in 3DWS into groups, lightmap, select a group, invert the selection, and delete everything else. Then export that single group as a .dbo file. Reopen the project and repeat with the other sections.
When you place the rooms, you'll place them all at the coordinates of the original level, because they'll be offset from the center already. No, no extra work for you.
You'll get a FPS increase since the engine won't have to render all of the level. It will render only the object that you see.
Come see the WIP!