This is a hint that allows to optimize drawing performance on some occasions.
Don’t issue this command multiple times between a Flip, it will degrade
performance or even cause undefined stimulus display!
You can time the execution of all drawing commands between the most recent Flip
and this command by setting the optional flag sync=1. In that case, telapsed is
the elapsed time at drawing completion. Don’t set the sync - Flag for real
experiments, it will degrade performance!
Some recent graphics cards provide a more fine-grained way to measure the time
spent drawing and processing your stimulus. See the help of ‘Screen
GetWindowInfo?’ about ‘infoType’ settings 5 and 6 on how to use that mechanism.
That method also has the advantage of measuring precise without degrading
###See also: Flip AsyncFlipBegin