Just a word of precaution. If there is a flow on, and you leave the laying box in too long before confining the queen, they will start storing nectar in them. I have actually had them start doing on within 24hr in rare cases. I also have noticed, the more the laying box is used, the less time it takes for her to start laying in it. So although the new cell cups have to be cleaned, I believe the smell/newness of the laying box is also a factor.
Is the box jam packed with bees? From what I understand, that is the key.
It is key that the cell builder be jammed with bees, not necessarily the mother donor hive. Though the more bees the quicker the cells will be clean. And filled with nectar too I guess. Perhaps a catch 22.