From my experience of at Passenger Agent in BFS we have had both free-seating airlines and allocated seating airlines, I have always found that having allocated seat usually is faster than free-seating.
Usually if a easyjet A/C comes on stand then 1 min later a bmibaby comes on stand beside each other, the bmibaby A/C will push back before the easyjet. Both aircraft would have the same capacity.
Richard