The last morning of a cruise looks empty on the itinerary and is the most tightly sequenced part of the trip. The flight booked for that day either fits the sequence or does not.
Clearance happens before anyone leaves
On arrival at the final port the ship must be cleared by border authorities before passengers may go ashore, and the timing depends on the authorities rather than the ship.
Clearance is granted for the whole vessel, so a delay affects every passenger equally regardless of when they intended to leave.
Ports handling several ships on the same morning process them in sequence, which is why arrival times cluster and why some ships clear later than others.
Luggage moves separately from people
Most passengers place bags outside the cabin the night before, and those bags are offloaded to a terminal hall sorted by tag color or number.
The offload has to be substantially complete before that group is called, because a passenger cannot collect a bag that is still aboard.
Carrying your own luggage off, where the ship permits it, removes this dependency entirely and is the single largest time saving available.
Groups are called in a controlled order
Passengers are assigned a departure group, and the terminal can only absorb so many people at once given the customs desks and the curb space outside.
Groups tied to transfers arranged by the ship are usually called in an order that matches coach departures, not passenger preference.
Requesting an early group is possible on most ships but has to be done during the voyage rather than on the morning itself.
The transfer is its own variable
Time from the terminal to the airport depends on traffic at an hour when several thousand people are leaving the same pier simultaneously.
Ports located well outside the city they serve add a fixed leg that no amount of early disembarkation shortens.
Airport check-in and security then apply as normal, and the terminal's queues are also being fed by the same wave of cruise passengers.
Building the margin correctly
Working backward from the flight, the sequence is transfer time, terminal exit, group call, luggage availability and clearance, each with its own uncertainty.
Booking an afternoon flight rather than a morning one absorbs all of it, and where an early departure is unavoidable, self-carrying luggage and an early group are the levers available.
Repositioning the whole problem by staying a night in the port city is what frequent cruisers do, since it converts a tight morning into an ordinary travel day.