Unreal Engine 1 games all have this problem under (at least) Windows XP, with (but not necessarily limited to) multiple core processors.
The solution here is to get the game running on only one processor, or only one virtual processor.
You can download a program like "windowsxplauncher" that can force any application to run on whatever processors you want, or simiply CTRL-ALT-DEL, right click on the application and select "go to process", then right click again and set the affinity to only one processor.
I have to do this with Deus Ex. Because I bought it over steam I can't use winxplauncher, and instead have to do this manually every time. But the program works for UT.
Any other solution - as far as I'm aware - will not truly bring the game to normal speed.