As it was originally reported on
https://bugzilla.gnome.org/show_bug.cgi?id=779234#c0, the hottest path was
convert_ubyte() in mesa. Reverting this shows no trace of those hot paths,
nor any higher than usual CPU activity.
As the improvements at the time were real, I can only conclude that pixel
conversion was happening somewhere further the pipeline, and swizzling just
helped indirectly. That got eventually fixed, so swizzling just stayed to
cause grief. And lots it caused.
Time to bin this, it seems.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/486
a871d56f88