I took the time to assemble a pretty responsive gtk/metacity setup for Gnome to reduce the drawing delay to more that just 50 or 60 fps from the usual Ubuntu themes. I used this benchmark table for gtk themes and also this one for metacity performance. I always preferred the classic Windows look, so it's no wonder that "Bluecurve" and "Bright" do the job for me to on both the aesthetical and performance side. It wasn't always like that. When I used Windows 2000 as a main system, I envied the guys having XP's look (I didn't install it for a number of reasons) but got quickly turned off as I was using it since the simple and classic interface was easier to grasp and performed better on my old machine. Looking at this benchmarks, it proves that those simple interface also give a minimal amount of screen delay you can also profit from (especially to check when an applications takes longer to react before redrawing).
As for icon rendering performance, simply choose one that's based on PNG files instead of SVG files, this way you get the fastest rendering performance. Font rendering is of course another major performance factor. It seems that most Linux fonts simply suck when rendering without any smoothing or blurring. I often use very small fonts, so using a non-smoothed setting does kind of destroy readability prettyness. That's why I prefer to leave the smoothing on, loosing a hefty bit of possible performance improvement.