The scheduled duration of many routes has grown over the years while the aircraft flying them have not got slower. The difference is padding built into the timetable.
Block time is a promise, not a measurement
The figure printed on a ticket is block time: gate departure to gate arrival. It includes taxi, queuing for the runway, holding on approach and taxi in at the far end.
Only part of that is flying. At a congested airport the ground portion can be a substantial share of the total, and it varies far more than the cruise does.
Airlines therefore schedule to a duration that absorbs a normal bad day rather than a perfect one.
Punctuality is measured against the schedule itself
On-time performance compares actual arrival to published arrival, so an airline can improve the statistic by moving the published figure rather than the actual one.
The incentive is real. Punctuality is reported publicly, sold in corporate contracts and used by travellers choosing between carriers on the same route.
A flight that lands at the same clock time as it did a decade ago can now be counted as early, which is a change in accounting rather than in operations.
Delay propagates through the day
An aircraft flies several sectors in a day and the crew is rostered around them. A morning delay follows the airframe to every subsequent leg unless something absorbs it.
Padding is that absorber. Slack in each block time and longer scheduled turnarounds let the network recover instead of compounding the problem into the evening.
Without it, a single weather event early in the day would leave the last departures hours behind and crews running out of legal duty hours.
The padding is not free
Aircraft are expensive assets and a schedule that flies them less each day needs more of them to carry the same passengers.
Crews are paid for time as well, and a longer published day consumes duty hours that could have covered another sector.
So airlines pad to the point where the reliability gained is worth the utilisation lost, and that point differs by route, season and airport.
What it looks like from the seat
Passengers experience padding as a cruise that ends early followed by a wait for a gate, or a taxi that seems to take longer than it should.
It also explains the announcement of an early arrival on a flight that pushed back late, which is otherwise hard to reconcile with the speed of the aircraft.
The schedule is doing the work, and it was written with the congestion already priced in.