I tried XFCE 4.6 on Xubuntu and Zenwalk and i must say the compositing is very slow compared to old one.:-|
I've an intel X3100 chipset.
I compiled it myself on Debian Lenny and could not witness such a regression.
lspci says Intel GM965/GL960.
However, with the old compositor, I would get garble-flicker when expanding menus, this seems to be resolved in 4.6.
A very good news is IMHO that running compiz on top of 4.6 no longer resets your virtual desktop count to 1.
Perhaps the compile switches got wrong in your installation?
I see this too, Debian Lenny, AMD64, Nvidia 8600gts graphics, Twinview setup and Xfce 4.6 from Corsacs repos.