(In reply to comment #3)
> Why don't we disable vsync by default?
> I think that's what proprietary drivers do.

Because that's relevant only for benchmarks.  Normal applications want to
present user complete (non-tearing) frames.

Note: with DRI2, using fullscreen with vsync off causes X to do copy of every
frame (which has clear performance penalty for memory bandwidth bound
benchmarks). With DRI3, when Vsync is disabled on client side, the X side copy
is done only every Vsync, but currently you would need quad buffering to avoid
stalling from X server side synching, which isn't what Mesa does (see bug
79715).

(If you're very close, but not quite at 60 or 30 FPS, this non-Vsync induced
copy can cause your FPS even to slightly decrease compared to Vsync.)