OK, I'll test stuff for you if you like.
I had a quick read through those threads, and tried some KDE apps. Scrolling was always smooth, and CPU time was always low (I managed to get User CPU load up to nearly 3% by wiggling a Konqueror window up and down like a madman
Athlon X2 4400+
nForce 4 SLI motherboard
2GB DDR-400 RAM
Gentoo Linux, KDE 3.5.6, kernel 2.6.20-g4, xorg.x11 7.2
nVidia driver 1.0.9755
Let me know what you want to see.
In case it's relevant, I use quite a bit of 2D CAD software for electronics, both under Linux and under Windows in vmware, and the performance is never an issue... then again, I don't really have high expectations, as the computer always seems to be faster than my thought processes