A performance critical fix was done on radeon linux kernel driver. It's not too invasive and has a massive effect for radeon Southern Islands and Sea Islands. Please, backport it in fedora 20 kernel. See upstream bug report.
Note they don't apply straight away to 3.12. They need minor massaging http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-3.13&id=9fadb352ed73edd7801a280b552d33a6040c8721 http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-3.13&id=439a1cfffe2c1a06e5a6394ccd5d18a8e89b15d3
Dave, should those go to stable? If not, should we bring them back to Fedora anyway?
*********** MASS BUG UPDATE ************** We apologize for the inconvenience. There is a large number of bugs to go through and several of them have gone stale. Due to this, we are doing a mass bug update across all of the Fedora 20 kernel bugs. Fedora 20 has now been rebased to 3.13.4-200.fc20. Please test this kernel update and let us know if you issue has been resolved or if it is still present with the newer kernel. If you experience different issues, please open a new bug report for those.
Sorry but I updated my fedora 20 to rawhide. All I can tell you is that with the 3.13.3-201.fc20.x86_64 and current rawhide mesa, the bug is gone.