[sandworm101]

Slowing down does not make an aircraft more fuel efficient. Best efficiency, as in fuel per passenger-mile, occurs with large aircraft at the very high altitude where air resistance is least. And to fly up there you need to move quickly. An a380, almost supersonic at mach 0.85, is more fuel efficient than Cessna at down low at mach 0.2. Even at lower altitudes, there is little advantage in flying slower. You just end up spending more time/energy keeping yourself up rather than moving forwards. A high-lift wing, great for maximizing time aloft, becomes a big draggy airbrake when you want to actually go somewhere.

In the extreme, the most efficient place to fly is in the most cold and dry air. That occurs at the tropopause, which is generally between 35,000 and 50,000 feet (-60*C). You don't get to play around at that altitude without a large complex aircraft.

https://www.windows2universe.org/earth/Atmosphere/tropospher...