It may be for tactical reasons. Moving slower gives the Borg's enemies more chances to shoot them. This might seem counter-intuitive, but the Borg must first be hit by the enemy's weaponry before they can adapt.
So they send in the drones slowly, the vanguards get hit and go down. This allows the rest of the drones coming in slowly from behind to adapt to the enemy's weapons.
If they have drones rushing to the front, they could all get huddled into one tight space. Then just one lucky grenade would be enough to kill the whole group. Sure, adapting would still take place, but an entirely new unit of drones would have to be deployed from the cube and rushed to the front lines, taking more time than was necessary.
Also, there's a reason for the Borg to act as frightening as possible. The Borg benefit the most from an enemy which has been routed, not one that has been decimated. If an enemy brakes formation, he/she/it can then be hunted down and converted into a new Borg drone.
A slow unstoppable anvil of force would be more frightening. It would give more time to contemplate their impending doom, and thus be tempted to break formation and run. It's all about maximizing the time for dread to set in.
This same strategy wouldn't work with human troops as they would also have to maintain their morale for this slow advance strategy, while Borg have no notion of morale.
It's also important to remember that every drone is both a warrior and a worker. Statistically speaking, they will spend most of their life as a drone in non-combat situations. And due to specialized drones being deployed evenly throughout the cube, there's no great impetus for having to quickly re-deploy workers to a remote location.