I'm seeing a similar issue, and I do use gnome. My impression is that it's not really a "speed of blitting" problem, but rather a small "lag" before each gui element/pixmap gets drawn. The severity of the slowness is dependent on how pixmap-heavy the theme is. If KDE is your normal DE of choice, I'm guessing you might have it more "prettified" than the default gnome desktop you've tested on for comparison. Try loading gnome with a more glitzy theme and see if it stays fast.
I'm wondering if this might be related to the bandwidth problem reported in: http://www.nvnews.net/vbulletin/showthread.php?t=84871
My current hardware is an ASUS EN8800GTX card in a Tyan K8WE motherboard, with 4 gigs of RAM and two Opteron 270 CPUs. I previously used a Leadtek 7900GTX, which had no speed issues. Output from the nvidia-bug-report on my system supplied for comparison.