Save more fuel by having the APU running instead of an engine for a long taxi ... to or from the runway ... *ducks*
In the air, high speed also means that turns have larger radii, so if there are significant turns, that can represent a fuel penalty.
If you are stuck at (say) 6000 feet, to clear the stack of another airfield, then accelerating may not only save you time but also get you out of a controller's hair more quickly, which may please him/her.