Regain
My understanding, no doubt somebody wiser will correct me, is that by flying below optimum altitude you would get there quicker; however, at the expense of burning more fuel.
Let's say - plucking out random figures here - two identical aircraft set out on a flight, ac A flies at 37,000' whilst ac B flies at 27,000'. Assuming they both fly at the same mach no, ac B will arrive at destination before ac A. However, ac A will have burnt less fuel for the same journey. All assuming still air, of course.