On 2003-11-15 17:21:08 -0800, (Jonas Heisenberg) said

So as I understand it, these planes are constructed to operate mos
economically at ca. 80-90% of their maximum speeds.

I don't know that they are designed and built that way. That's just the way it works out

Has it ever bee
tried to build planes that are built specifically for lower speeds an
higher efficiencies (recent - no WWII)

http://www.nasm.edu/nasm/aero/aircraft/rutanvoy.ht