VNAV expects a continuous deceleration to the proposed flap setting when coming back to clean configuration.
A seasoned aviator will roughly know how long it takes for flaps to extend, thus ensure you select the next flap setting required for phase of flight with that time in hand, e.g. approximately clean speed +20kts works reasonably well to get flap 1 and immediately afterwards 5 out in time for approach segment, depending on angle and weight flap 10 may be required to maintain speed on the subsequent final descent angle coded.
Think of it that thrust levers stay idle as long as possible until throttling up to maintain speed on intermediate approach segment before gear goes down and further deceleration takes place...
Unfortunately I have yet to find references to the question you're asking...