400Mhz isn't below spec. And there isn't anyway to tweak the game beyond what you have done. Have you tried tweaking your computer. Depending on the version of Windows you have, you may have alot of background clutter and mermory leaks. Are your drivers upto date, most importantly your video driver.
I need more info on your system. How much RAM, what size HDD, what version of Win? What else is running when you play? Most computer users have no idea what useless garbage is running in the background, and how much system resources are wasted.