Hi Benoit,
Do you still reproduce your problems with compiz 0.5.0 currently in
unstable? With xserver-xorg-core 1.3 and latest ATI drivers (6.6.192 is
currenlty in experimental)? To reduce speed issues, you might want to
try adding
Option "XAANoOffscreenPixmaps"
to the device section of your xorg.conf.
Brice