I've found that KDE4 is very slow by default on Intel graphics hardware. However, this can be fixed with three lines in xorg.conf:
Section "Extensions"Note that this will probably make OpenGL effects not work.
Option "Composite" "Disable"
That's kind of the point actually... But still.
My understanding (probably somewhat incorrect) of why this is: apparently, KDE4 has some eyecandy or eyecandy-enabling stuff that can't be turned off through the configuration utility. This functionality is very slow on certain hardware. But it uses Composite, and if you disable that, KDE won't try to use it.
Anyway, now that I've done this the difference is very visible; on my Aspire 3680 (1.5 GB RAM, 1.6 GHz single-core Celeron M 520) KDE went from being laggy and unusable to being as fast as Xfce.
I'm somewhat amazed. And a bit ashamed of the jabs I've made at KDE for being sluggish, seeing as it now runs like greased lightning.