After playing around with Debian “wheezy” and getting fed up with X.org 1.10.x, I decided to go back to Debian stable — but not without trying downgrading to X.org server 1.9.2 from Debian Experimental 2010-11-31 first!
It turns out that yes, most of the slowness and pre-paint garbage were caused by the X.org server alone. Sadly, the intermittent shadow glitches are seemingly inherent to KDE SC 4.6 and not to the graphics server.
Annoyed and without the required willpower to file bugs and wait for things to be magically fixed in a few months, I burned a Grml live CD from an ISO I downloaded for no particular reason at all about a week ago (!) and embarked on a journey to wipe out my root file system and restore Squeeze from the pre-ugprade backup in my external hard disk.
It was a successful mision, barring the ensuing breakage caused by the inconsistent versions of GRUB found in
/boot and the disk’s MBR.
update-grub later, Reicore was running Debian GNU/Linux 6.0.1 again, at last. In total, I spent around an hour on the restoration procedure. Cheap.