Wind speed increases as the distance off the water
increases up to a couple hundred feet off the deck
. So not only is the high aspect ratio Marconi Rig more efficient but it also has more wind available to suck the boat along in ligher air conditions.
Think sail material also had a lot to do with the near universal adoption of the Marconi Rig. Cotton sails
were heavy, even heavier when wet and stretched badly when pull was across the diagonal of the threads. Sail makers compensated for the stretch with a miter cut on the jibs. On the main, a gaff sail doesn't have the diagonal forces that a Marconi main does. Dacron and other synthetics are far less affected by diagonal, is that called bias, stress on the sails. Most dacron sails use relatively horizontal panels
because the material can handle the diagonal stretch better. It's also way easier to make a sail that way. Not to say that sailmakers don't still use the miter cut to minimize diagonal stretch especially in racing
The slot between the jib and main also increases sail efficiency especially on a reach. I used to sheet the Reacher/
Drifter to the main boom on a reach. By opening up the slot between the two sails, would pick up a 1/2 knot
If you haven't sailed a small boat with cotton sails, you probably don't appreciate how much the weight of wet sails made on the stablity of a boat. A high aspect ratio rig with cotton sails would have been/is dramatically less stable than with dry sails. The short gaff rig isn't as affected by increased weight aloft.