Give this a try, by adding it to your ServerLayout section of your xorg.conf file.
Option "BackingStore" "true"
Supposedly it should help with some graphic performance, with things like transparency or accelerated effects. It will depend on your video card of course, but for my Geforce 440 Go, it seems to work okay. Or at least, it doesn’t make things worse.
Found while perusing the Gentoo wiki … which is back in business, thank goodness. All is right with the world again.