Saturn has posted some good suggestions for this, here's what they have listed previously:
1) Turn off global illumination
2) Turn off shadows
3) Turn off ambient occlusion
4) If you have NPCs on your map, make sure their behaviors have movement intervals of something like 1.5 or greater. Short intervals tend to have a huge impact (0.5 or less can be pretty impacting).
Also using less complex models, for example marching cubes on large, multifaceted model that you use alot can hinder performance. Remove unnecessary nav lines, lots of animated tiles can also cause performance drops as well.
