Disney Cruise does not tell anyone to show up at 10:30. Pearl guests may enter the terminal as soon as they arrive, but they are not required to arrive at any specific time as long as they are onboard by mid-afternoon. Guests taking 
DCL transportation are issued a port arrival time that should allow them to enter the terminal as soon as they arrive, but that does depend on how many people are on each bus and how many buses there are.
DCL typically opens boarding at noon or later. People who don’t want to wait long in the terminal can choose to arrive at 1 pm or later, when they will usually be able to board shortly after arriving.
I haven’t sailed on any other cruise line in almost 30 years, so I don’t know how boarding works on other lines. However, it seems reasonable that it takes 3-4 hours to fully clean and prepare the ship between when previous guests leave and new guests board. That is shorter than the turnaround time for Walt Disney World resorts, where just a fraction of rooms turn over each day.