This sounds an awful lot like bug 36666 (but for a different card and driver). I'll repeat what I said there here:
"For fastest performance a driver has to implement GLX_EXT_texture_from_pixmap (I may have the exact name of the extension wrong) otherwise this rather critical feature will have to be emulated in software. [...] Xgl is so new and requires such cutting edge features that problems like these are to be expected. (This bug should thus not be filed against xgl but instead against the xorg drivers for your card)" -- poor desktop performance on compiz https://launchpad.net/bugs/46119 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
