DaedhirUI for World of Warcraft v3.1.
Get it here.
This is a much more lightweight version of the UI than the previous ones. It should be much faster, and it requires less memory. As far as I can tell, any slowdown in 25-man raids should be gone.
It's probably the best idea to reinstall the UI from scratch. I did a complete overhaul and the UI scaling is slightly different from the way I had it. I hope the instructions are clear enough. They are enclosed with the UI.
Hope everybody enjoys this, it's working great for me in Ulduar.