Peter,
the objective is of course to fully fill and empty the waterballast tank as quickly and as conveniently as possible.
Filling, the water flows in with up to 2 litres per second, so the tank should be able to vent air at the same pace. Just to loosen the main hatch is not enough for that flow rate.
During emptying the air has to be sucked in at a high rate, and there a just loosened hatch cover tends to be sucked down and self-seal, further reducing the venting flow rate.