One of the problems for us pilots is that the bean counters analyse fuel burn on a block hour basis. In other words, the amount of fuel burnt per hour.
If three identical aircraft flew from London to Frankfurt, one flew fast and did it in 50 minutes, another flew at LRC and did it in an hour, and the third aircraft took 75 minutes at minimum speed. If all three aircraft used the same amount of fuel e.g two tonnes, then the bean counters would show the third aircraft was flown the most efficiently.
How? The first aircraft's block hour rate was two tonnes in fifty minutes, and that equates to ablock hour rate of 2400kg/hr. THe second aircraft's block hour rate is 2000kg/hr, whilst the third aircrafts block hour rate is an incrediblly efficient 1600kg/hr.
So even if the first aircraft only burnt 1800 kg in doing such a flight in 50 minutes, the beancounters would show this as being less efficient than doing the flight slower and burning more fuel.