No windsurfers opt for inefficient mesh-paneled sails for the purpose of going slower. Competitors want the fastest gear they can get. Superb response to the wind means more performance and more fun . Exciting responsiveness to the wind is not considered important. Good edge to power-zone acceleration, and good speed response to the wind, are bred out of competition kites to make precision, pairs, and team flying possible.
Good exercise come from force with movement.
That's why walking, running, bicycling, skiing, and skating are such good exercise. With most dual-line kites, the pull drops when you turn, the opposite of what you need for good exercise. Little force and little movement provide little exercise. WindDancing provides wonderful exercise. The pull rises when you turn.