http://www.youtube.com/watch?v=Oix6sHKzOLU
What seems to be the case...
Altitude and speed are interchangeable and the plane which makes that exchange most efficiently appears to be a glider, i.e. the slipperiest possible plane with nothing whatever to catch the air or produce drag. A glider can get as high as anything else on updrafts and thermals and then start negotiating exchange rates for speed from altitude.