I've hit this in Arch Linux and it's quite annoying: Open-Source GPU Drivers Causing Headaches In KDE 4.5-series. I was angry at first but then I read Martin's explanation for what was happening and decided to be more forgiving. I am experiencing this on an old system with a POC integrated video card after all; the fact that I get any effects at all is somewhat admirable.
It was pretty disappointing however that this problem
exists. I didn't read them but something like this should definitely be
in KDE's release announcement. Why? So my distro (who's announcements I
am likely to read - after something goes wrong) can point me to it.
Then I wouldn't have a) upgraded or b) had such a hard time figuring out
why X was crashing, and why Desktop Effects were so PAINFULLY slow,
after upgrading Xorg. I didn't ever really think it was wholly KDE's
fault which I suppose helps with being forgiving AND I was being
stubborn by trying to force desktop effects on through XRender after
multiple crashes. After all It worked before so why not now?
Worked
there being a relative term however.
Anyhoo Backend=XRender and OpenGLIsSafe=true in my $KDEHOME/share/config/kwinrc
file brought me back to the painfully slow compositing, rather than
crashing straight to GDM repeatedly, which I thing I tracked to kded and
dbus having a lover's quarrel (YAY!). After getting the compositing
back I turned off everything unecessary like transparent windows and
BLUR. I only really want transparency in specific apps (Terminal,
Plasma) and the blurred Oxygen window highlight.