First of all, I see no pics attached, so if you get those up I'd love to take a look at the possible problems.
But, well there are always certain things that can cause severe lag: a big one is enemies. If you have tons of enemies spawned at the ready in the map, that will significantly lower the FPS.
Another, if you have a lot of dynamic entities, that will slow down the game as well.
And then the obvious of course, depending on how
large the actual level is, can cause lag, along with any doors that you may have in the game that lead nowhere. Always best to put in a single segment past the doors you can't get through, that way the "Build Portals" part of the loading doesn't take as long, and the game should run a little faster.
Again, hard to tell what the problem might actually be - it could in fact be none of the above - without any pictures.
-Cheers
Ich will in Beifall untergehen.