This is a fix found on the Bugtraq site for the Unureal Tourname 2004 Demo. There seems to be a huge performance issue with the audio system in the UT2004 Demo that absolutely kills frame rates on single processor machines. To turn off audio on the demo, edit the UT2004.ini file located in you user's Library -> Application Support -> Unreal Tournament 2004 Demo -> System folder. Change the UseSound argument from True to False.
The dmeo went from unplayable (about four frames per second on a G4 733 with a Geforce3 and 768MB RAM) to completely playable with default setting @ 800x600 (sans audio). No slowdowns, stutters, or perceivable screen tears. Even had shiny, reflective water.
[robg adds: I can't confirm this one, so if anyone else can, please share your experience...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040224134354973