Speed applies in two ways. The muscular requirements of sprinting and jumping are very close, there being a high a high correlation between raw speed and raw jumping ability. Plus the more controllable speed (horizontal velocity) one can bring to the HJ takeoff, the more lift (vertical velocity) one can generate. All that said, more speed does not always translate into more height. In Dwight's day they make a distinction between a speed flopper and a power flopper. There was a guy named Noji I believe, not very tall, who sprinted into the takeoff, had a big arc and still made around 7'7 (don't have any facts before me). Others appear to jog to the bar and drive straight up.