Once you're up on the foil, the length of the mast isn't really relevant to your speed on the water. In theory, a longer mast does weigh more which could slow you down and if you have more of the mast in the water then you'll have more drag, but you can control how much of your mast is in the water. Speed is more determined by the front wing of the fin than the mast length. A front wing with less thickness, a smaller surface area, and less angle of attack will result in more speed for the rider.