In a word: Drag. V-drive boats are inboards, which means they have two shafts, two struts, two props, and two rudders beneath the boat, all creating more drag than an I/O's sleek lower unit. Although the ZF 63-IV drives produce a relatively shallow 12-degree shaft angle, they can't be as efficient as a stern drive, which has no fixed shaft angle and can direct all of its thrust in the desired direction - forward. At high speeds, stern drives can be trimmed up. This lifts the bow slightly, taking some of the boat out of the water, which further reduces drag.