I'm guessing they will have a cart where they can stack a few bags and then one (or better two) staff to store the carry-on before passengers board.
If properly pre-sorted and done methidically from front to back this could speed up boarding.
But, on ghe other hand, if they have got all that time why not start boarding earlier? I remember that (years ago) CP, on one of thier short lived all business products had 'trickle' boarding. They opened boarding 45 mins before departure and pax could board as and when they chose. Seemed almost civilised.