Generally, if you can generate lift at low speeds, you generate more lift when you're going fast. At some point, that extra lift just becomes a drag to your plane.
Consider a current jetliner. They employ very complex flap systems to alter the shape of the wing to increase its lift for takeoff and landing. A larger, squarer wingspan will generate more lift, but that lift becomes an issue at higher speeds. A smaller wing cross-section infers a weaker wing, which also limits the lift a wing can generate.
A 10% hit in airspeed is worth a 50-70% gain in fuel efficiency. Don't expect to see this reflected in your ticket price ;-)