Every airport publishes a minimum connection time, and booking systems refuse to sell an itinerary below it. The figure is calculated from measured processes rather than estimated.
What the number is actually measuring
The clock runs from the scheduled arrival of the first aircraft to the scheduled departure of the second, which is a longer period than the passenger experiences as walking.
It absorbs taxi time from runway to stand, waiting for a jet bridge or a bus, and the time an aircraft spends parked before doors open.
Only after that does the passenger's own journey through the terminal begin, and the published figure has to cover the slowest realistic version of it.
Terminal geometry dominates the figure
Airports built as a single concourse can move passengers between gates quickly. Airports assembled from separate terminals over decades cannot.
Where a transfer requires a train, a bus across the apron, or leaving and re-entering a secure area, the minimum rises steeply.
This is why the same airport publishes different minimums for different terminal pairings, and why domestic-to-international transfers are usually the longest.
Border and security processes add layers
A transfer that stays within one security regime can happen entirely airside. One that crosses between regimes requires re-screening, and sometimes immigration control.
Some countries require all arriving passengers to clear immigration and customs regardless of onward travel, then check in again for the next sector.
Those requirements are set by governments rather than airports, which is why certain hubs have long minimums that no amount of terminal design can reduce.
Baggage moves on a separate timetable
Checked bags travel through sorting systems that have their own throughput limits, and a bag transferring between airlines may need manual handling.
The minimum connection time has to be long enough for the bag as well as the passenger, and the bag is usually the slower of the two.
This is why a passenger sometimes makes a tight connection while the luggage does not, and arrives on the next available service.
Why booking below the minimum matters
A connection sold as a single itinerary carries an obligation: if the first leg runs late, the airline rebooks the passenger at no additional fare.
Two separately purchased tickets carry no such link, even through the same airport. A missed second flight is simply a missed flight.
The published minimum is also a legal minimum, not a comfortable one. It assumes both flights are on time and nothing in the terminal is congested.