Using a "sync delay" where we wait for 2 ms after the vblank before
starting to draw the next frame provides for much more predictable
latency for applications. An application can know that if it completes
a frame any time between 8ms before the vblank to the vblank,
it will reliably be drawn on the following vblank period, rather than
having an unpredictable latency depending on whether the compositor
is currently busy drawing a frame or not.

https://bugzilla.gnome.org/show_bug.cgi?id=685463