Have to agree with Notso here. There's no point asking someone to slow down unless it's likely that the hold will be clear by the time they get there.
If you are continuing to feed traffic in from all directions, then let them come at their own pace, unless of course the speed is to maintain separation from someone ahead.
Often it's much easier to let them go in to the hold for once around the buoy than start vectoring and speed controlling a multitude of aircraft with the increased workload and RT loading which that brings.
Having said that, I would expect the pilot to comply with any SLPs or published holding speeds as normal to ensure that they remain within the protected airspace of the holding pattern.