I've heard the metric extend to about 16 seconds average.
Something about the ability of the brain to maintan a sense of timing on a pitch with the argument being that an adjustment in speed in more difficult for the batter if he does not have sufficient time to clear his head. To wit, after a 92 mph fastball, the brain gears to hit the next 92 mph fastball. If he sees a 84 mph slider, before enough time passes he simply does not adjust well.
I'm sure there are a million of exceptions and not all pitchers are even comfortable working so fast. Still it's all fascinating baseball and brain science to me. What made this discussion even more fun was I had it with Vasil's former pitching coach during a Vasil start so I was paying very close attention.