Unless you are on a speed control instruction then, providing you can make the 'level by .....' restriction, you can fly as slow or as quick as you like. I think that the "when level reduce to holding speed" instruction is a habit which has come about by pilots requesting to slow down and controllers trying to be helpfull in reducing the time spent at the holding fix, when infact the speed decision is completely in the hands of the pilot and does not need to be mentioned to the controller.