Speed control is used purely for sequencing, it's not an intrinsic part of flying a CDA from an ATC point of view (imo). I don't reduce the speed of traffic in the hope of affecting its rate of descent, it's up to the pilot how he flies or configures to achieve the requested speed.
If there's one a/c inbound, or a group widely spaced, let them manage their own speed and just give them an accurate distance to go to allow them to manage the vertical profile for as far as is practicable to achieve a CDA.