Greg, I think it is more than the cruiseline trying to save money by leaving the port in the late afternoon. It also has to do with dockworkers and the time it takes to sail to the next port. In other words, if your ship does a top speed of 22 knots, that is a bit over 25 miles per hour. They do have a scheduled arrival time at the next port where the pilot and dockworkers are standing by to receive the ship. If you plan your port activities right, 6 - 8 hours in port will allow you to enjoy your visit.