Hello,
I'm on Vista and currently UT feels like I'm running through treacle.
I tried changing the power management settings but this has not worked. I was wondering, since GOG mentioned that they turned on single core for XP on by default, is there a way to fix this issue only for Vista users?
Many thanks.

Yes, go to this post and grab one of the patches (OMP-UGOLD or 227.e).

I have in fact managed to fix it, but I'm not entirely sure how.
I couldn't manage to get the patch installer to recognise my installation of UT but if you create a new shortcut directly to the UnrealTournament.exe executable and add -CPUSPEED=xxxx as an argument, where 2,4GHz would be 2400, it might just work.
Again, still not sure how I solved it and it's a bit jittery at times but it runs at normal speed.

Oh, crap I'm sorry this was for Unreal I misread you.
You can try one of these renderers for UT, it could solve your problem.

The OpenGL renderers + the CPUSPEED argument work. The game is still jittery at times but it's very punctual and otherwise the game flows like a charm.
Thanks, DukeNico!

http://www.gamedev.net/community/forums/topic.asp?topic_id=353685&whichpage=1&#2317294
this post described a solution that worked, in combination with the CPUSPEED argument, for me. The lower posts talking about it being slighly dangerous but it shouldn't be any more dangerous then encoding a video file. Just close the program after you are done playing and you're back to normal.

Run it in Windows 98 compatibility mode.
Right click the shortcut, go to Properties, click the Compatibility tab, select Windows 98 from the drop-down menu. Click Ok, and then launch the game from the shortcut.

Doesn't work for me... it runs faster but after sometime it slows down again. Thanks anyway. I don't know what to do.

I remember this problem on XP. The problem I had was SpeedStep (well something similar) on the Pentium M CPU I was using at the time. The solution was to load up something CPU hogging (like another game), and then load UT99 after it. I don't know why, but if I didn't the game would run superfast. Never tried the other graphics renderers, they weren't an option in 2005. First I heard of the CPUSPEED parameter also.
Wait, there was a program, SpeedSwitchXP that I could use to control the speed of the CPU. Try that on Vista. You'll likely have to run it in Admin mode. Since it hasn't been updated in 4 years, I'd expect that. I would throttle the speed to max, then load the game and it ran as smooth as I remembered on my old Celeron 300a (overclocked to 450MHz)/3dfx glide at 4 times the resolution. Speedfan is another nice utility, but it won't throttle your CPU - mainly it just throttles your fan, if you have a variable speed one.
Hope this info helps.