I believe that Rideman explained this before, and he said that the lift speed doesn't really effect it's initial maximum speed. I think the reason the lift speeds up is because the third train has passed it's block by moving in to the unloading station, so the computer knows it's safe to launch the train.
It gets up to speed at the bottom of the hill, and all you can hear is the wind whipping past your ears (as in the POV on the official site) and the yelling and screaming of exciting passengers.