But it won't.

Much like a car's city vs. highway mpg (though for different reasons), a plane is most efficient at its design cruise speed. Jet engines operate more efficiently at greater velocity differential, the trade-off of induced vs. parasitic drag reaches a minimum at cruise velocity, etc.


Either slower or faster reduces fuel economy. Note the decreasing percentages in all directions away from optimal cruise.



Unless they believe the plane is going to have to burn fuel by circling, traveling at efficient cruise and landing early will burn less fuel than going slower and getting there on time.