Most people who depend on screen readers prefer the synthetic voices for their consistency which can allow them "read" at far faster speeds. I've been working for a couple of years to reach the speed one of my friend regularly works at, about 900 WPM. I topped out around 550 WPM with the human-sounding voices and then almost immediately got to 650 moving to a synthetic voices. Still a long way to go to hit 900, but I'm certain it's possible, and nearly certain it's impossible with text that's voiced to sound human-like.