
Being a BeOS user (a purely desktop system) and because I code under Linux, I see
XFree86 (v4.1 on my machine) as a user and as a developper. And this is where the problem lies. My Gnome or KDE desktops are slow in comparison with other operating systems, but XFree86, the 'engine' behind these desktops, proves me that it's not. Let's look at what I have in front of me: a dual Pentium III at 933Mhz with 512MB of memory, a Radeon 32 AIW, a modified Mandrake 8.0 powered by kernel 2.4.18.
I am here with a much more slower PC than yours (dual Celeron 533, Voodoo5) and I don't see any of your problems with XP. I think your problem is buggy drivers, or drivers not interacting well with your PC.
These are two token systems from which I've taken screen shots. I can certainly take more... these problems are present on every XP system I've used which uses any theme with window bitmasks.