You program the FMS [Boeing]with scheduled descent speeds and forecast winds based on the planned profile often constrained by ATC eg. MARGO FL260- for ScTMA landers. All that produces a Top of Descent point [TOD] where ideally the thrust levers retard to flight idle & you start the descent.
Asking for an increase in speed results in an increase in the rate of descent [ROD], and a profile below the ideal "slope". Costs money by using more fuel in the lower levels when thrust applied.
Asking to slow down reduces the ROD, and therefore any ATC restrictions can now only be achieved by using the speed brake which increases the ROD to a certain extent only. Function of IAS. Using the gear to increase drag would be a really bad day !
The NG wing is designed to be efficient and "glides" well.
If say, a speed reduction is necessary, try to inform the crew before TOD so the desired speeds can be entered into the FMS to produce a modified TOD. Late reductions in speed are also bad news. Shows a lack of forward planning & a lack of understanding of aircraft performance.
Answer : arrange jumpseat rides and see what happens aloft.
