Well, the fact that its a BSP level is one problem. BSP runs substantially slower than modeled levels.

LOD is the easiest way to increase FPS, but only in wide spaces. If your in a small room, LOD will likely do nothing for your FPS.

Really, if you have small, indoor levels, and you want to increase FPS, you will have to either decrease some texture sizes, or get REALLY creative with your level design.

If you could design your levels so that the player only sees the bare minimum of a level at any given time, you could increase FPS. However, this is a very difficult task to achieve, especially while keeping the game interesting at the same time.

Anyways, good luck,
Raddaddy