Many people do not realize that the ship they are getting ready to board must unload the passengers who just took the same cruise they are taking. The lines at the terminal begin forming sometimes before the current passengers have even begun to disembark. 

If there are stragglers left on the ship, they will not begin loading until every passenger has gotten off and this can be time consuming if some fool is trying to squeeze an extra hour in on the ship.

Dropping luggage off early is fine but getting in line is a waste of time.