EXA is a new 2d acceleration architecture introduced in xorg 6.9/7.0. For nouveau, EXA is enabled by default; there is no support for XAA.
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EXA performance (especially with composite on) seems to improve with: (without it, an idle composite extension seems to negative effect on cpu usage)
Section "Device" in /etc/X11/xorg.conf: [[!format txt """ Option "MigrationHeuristic" "smart" """]] Things like transparancy is still dirt slow though. Time will tell if this really helps.