The engine just needs a bit of optimising, that's all.
Since I don't own FPSC myself, I can't test it out, but try this:
(Load up Fraps or another on-screen FPS counter for a bit later in the experiment)
Make a wall, texture it if you wish.
Run the game, and walk up to the wall so that the wall fills the vision.
Note the FPS rate.
Now, go back to the editor, and load a complex level (preferably without AI), and insert the same wall.
Again, run the game, and walk into the wall until it obscures your vision of anything else.
If the FPS rate now is different from the one before, then do not worry, as soon as the/a culling algorithm is fixed/improved/made, the game will speed up a great deal
Also, with the AI, try making them so that they don't activate unless you're within a reasonable distance of them - that should speed up the game, too.
How's my typing? Phone 0800-GO-TO-HELL