I have compiled all the infomation I know about increasing performance in one thread:

First read the FPS Creator FAQs!

There are many tips regarding performance and common errors.
http://www.fpscreator.com/faq_tech.html
Extra tips
- Use trigger zones to dynamically load and unload entities. (Please use the search function to look for a tutorial. I have seen so many posts on these tutorials.)Yes, even companies such as Activision use scripted events to make massive game levels.
- Go to FPSC Game (in your FPSC folder) and go to Properties, on the compatibility tab click run in 640X 480 resolution. (This is mainly if you have a very bad GPU such as intel where the difference is significant.)
- Use less dynamic entities.They are the performance hogs.
- Use a performance mod. Some mods have made FPSC faster but add even more features that counteract this speed increase. If you don't need fancy effects, disable them.
- FPSC Version 1.09. They have addressed many of the performance issues. The newer versions have done the same but they tried to put more content in it.
- If you're desperate, use a template entity to test your game. A simple entity with no texture.
NOTE: The main problem with FPSC is requiring a good GPU. In the case of game making a good GPU will be more reliable than a good CPU. (While you're playing the game. The GPU does more hard work than the CPU)
These tips should dramatically increase performance.

(I got 200% faster running test game this way.)
I'll add more as I go back to my older threads.
If all else fails upgrade or buy a new computer.
Happy game making.