From JVN's Acceleration calculator, I've found a speed of around 16 ft/s gains the largest time loss to travel a set distance when using 6 motors instead of 4 (Shaves off about a half a second on average). As you get further from 16 ft/s in either direction, the time difference to travel a set distance between 6 motor drives and 4 motor drives gets closer and closer to 0.