I realize this is isn't the most tantalizing topic, but I finally turned off the default Windows XP Theme "Luna" today. I did this because I was getting some weird color drawing behavior in a custom app, so I wanted to see what it looked like in Classic, but what I noticed immediately was that the UI immediately felt like it was responding faster. I don't know if it is benchmark true, but it feels true so I just might leave it off for good. I mean, why bother putting lipstick on a pig, its still a pig.
