This only works on Windows XP (maybe 2000). It sped my game up a lot.
When CM4 is running, press Ctrl+Alt+Delete, choose the "Process" tab, right click on cm4.exe, and set priority to "above normal".
You probably shouldn’t go higher than that. The downside it that other applications will run slower. (not recommended if you run Winamp, Kazaa or Internet at the same time as CM4)