I would think thicker would be be less drag and faster as long as it is not wider than the aft edge of the keel. Matching the width of the keel exactly seems like it would be less drag. But then tapering would be more complicated. Another issue is the trailing edge. Some have made it square rather than pointed touting speed benefits. The production rudders you can buy are square tailed, yet none of these have won in nationals ,so might not have an advantage over a carefully hand shaped tapered rudder. Then a few have made a slight concave in the trailing edge with sharper edges again looking for more speed. I have no idea which method is the fastest