Like somebody said earlier - in float workports A&B are connected to tank but it is upto the valve designer and machine manufacturer how much the ports are open to tank and therefore during the development process trials are done to check the rate of drop during float and then the amount of opening is increased or decreased. this means there is no extra restriction other than the spool design itself. during lower of a heavy load in this manner you could (as somebody already said) cavitate the non-load holding side of the cylinder.