Alhtough I made KDE look like pile of shit, it felt faster than Gnome, mostly because the renderer were faster. I think it's due to the fact that GTK does some different things and allows more complex and performance-eating structures. Under Gnome, I simply switched to the theme "Raleigh" and everything worked a bit quicker. Raleigh doesn't use any theme engines or so, just the basic built-in one. Thus, it may depend on which theme you use and what theme engine it requires to run. Whatever, as long as it fast enough to scroll large areas of gadgets, I don't think it's necessary to even think about such stuff. Window creation, scrolling and resizing can differ from system to system and always depends on the applications to resize their contents quickly. I always had performance problems on my system, but on a clean Debian 5 it's enjoyable. Maybe it's really not matter of the rendering, but a matter of the intervall, GTK takes to draw stuff. I don't know. I can't measure it 'cause I don't have another system to test. Independent from how fast it actually is - I somehow tend to like it's rather slow and sticky beeing. It's like comparing the controls of Stalker and Metro 2033: Metro 2033 is more direct and quick, Stalker rather floaty and sometimes lagging. Or like a comparison between the controls of Super Mario Sunshine and Beyond Good and Evil. The latter ones are always slower and response later, but this requires less quick reactions from you. Even if Gnome's GTK feels heavier and slower, it also feels more comfortable and more controlled than KDE's Qt.
Also, KDE's icons are ulgly, too. Gnome icons are crisp and nicely shaped - perfect to remember. I can't say this about KDE.