For a given mach number in still air,the higher aircraft will be slower.The highest aircraft of a given type will want to descend later cos its probably lighter.mach0.8=8miles per minute,0.7=7miles per minute.At these speeds the folk in the airplane would appreciate if you would pause for breath between transmissions,so they can get a request in before overflying the destination.If you want to get on in Air Traffic Control please ignore the above;It's worked for all your predecessors.
Good luck guys. (It might be the second best job in aviation.)