slowing down 5% adds 10 minutes flying time to a 3 hour flight. Drag varies with square of speed except when you get close to Mach 1 where it gets much worse.

Its also possible to fly 20% (36kft vs 30k) higher where drag for the same speed will be lower. Drag varies linearly with density.

An aircraft purposely designed for low fuel consumption could save 50% or more jet fuel without being significantly slower.

https://en.wikipedia.org/wiki/Aurora_D8

https://www.nasa.gov/topics/aeronautics/features/future_airp...