I have been fighting with what I thought was a really slow window manager, and so I changed to a lighter weight one and it still took forever to draw things. After fiddling with stuff on and off for a few months, it turns out to be pretty simple: the radeon driver decided to use the CPU for too much of its own job.
I set a couple of flags (thanks to linportal), and everything is speedy again. So if you're fighting with a radeon driver that seems to be worthless (especially with multiple displays) try setting the "MigrationHeuristic" option to "greedy" in your xorg.conf's device section.
Option "MigrationHeuristic" "greedy"
