I don't know that that is the case. Remember that for every downstroke, you have to come back up again before you go back down, so it is in effect 'alternate picking' at least as far as motion and direction.
In most cases, speed is determined by technique; hand position, arm position, movement of various joints, pick position, etc...and you can develop slightly different techniques for alternate picking vs. all downstrokes in order to do each style best.