Added time at the end of each half has grown substantially. The change came from how the time is calculated rather than from matches becoming slower.
The original allowance was an estimate
For decades the fourth official announced a round number that reflected the referee's impression of how much play had been lost.
That impression was conservative. Long stoppages were counted, but the many small ones were absorbed into a general sense of the game.
Because the number was a judgement rather than a sum, it clustered around familiar values regardless of what had actually happened.
Counting replaced estimating
Refereeing instructions shifted towards measuring specific categories of delay: goal celebrations, substitutions, injury treatment, penalties, and video review.
Each of these takes longer than intuition suggests. A goal followed by a celebration and a restart can consume more than a minute, and a match with several goals accumulates that repeatedly.
Adding the categories together produces figures that look extreme next to the old estimates but describe the same matches.
Substitutions became a larger factor
Squads were permitted more changes per match, and each change involves a walk to the touchline that a departing player has little reason to hurry.
With more substitutions available, the total time spent on them rose sharply, and it rose most in the closing stages when a leading team wants the clock to run.
Counting that time directly removes the advantage of using it, which is part of the purpose.
The effect on how matches are played
Longer added time changes late-game behaviour. A team behind knows that a period of real length remains, so it commits players forward earlier and with more structure.
A team ahead can no longer close out a match by slowing it down, because the slowing is added back.
The result is more genuine playing time and, by extension, more late goals, which alters how points accumulate across a season.
The load it creates is real
Extra minutes at maximum intensity accumulate across a congested calendar, and players and coaches have raised the physical cost.
The alternative under discussion is a stopped clock, which would remove the estimate entirely and end the argument about accuracy.
That change carries consequences for broadcast slots and for the rhythm of the game, which is why it has been trialled cautiously rather than adopted.