If your thrust is idle then your fuel flow is the same regardless of whether or not you fly fast or slow. What changes is the time spent in cruise and the fuel burn in the cruise. If you descend at an earlier point at min drag type speeds then you have spent less time in the cruise and therefore burnt less fuel. If you descend at high speeds ( ias) then you have a steeper descent profile which means that you have flown closer to the field which in turn means you have spent longer in the cruise and burnt more fuel. how much depends on type. 737 about 40kg /nm cruise. 747 at end of cruise about 20 kg /nm. On a practical level if you are too slow or fast compared to other jet traffic your vectors will eat up any savings as atc fit you in behind others.