Efficiency actually increases the faster you go at some point until a certain limit due to being able to ride the 'boom' turning the energy required to keep the

Efficiency actually increases the faster you go at some point until a certain limit due to being able to ride the 'boom' turning the energy required to keep the plane up into energy that pushes you forward, there's also special things that happen to friction when you pierce the barrier which are still being researched. Flying higher also means less molecules hitting your cruiser.