From what I understood from the two videos below, it seems the reason for our Lord Buddha’s relatively short lifespan—compared to other Lord Buddhas—is because he was born as a “Pragñāvantha Bodhisattva.” From my understanding, “Pragñāvantha Bodhisattvas” can be born during periods when the human lifespan is short. However, “Sandhātika Bodhisattvas” are only born during times when the human lifespan is long.
The videos state that when a Buddha attains enlightenment, over 20 incalculable numbers of beings (worldlings) will achieve maggaphala. Therefore, during the lifetime of a “Sandhātika Bodhisattva,” all those 20 incalculable beings are able to attain maggaphala while that Buddha is still alive. After that Buddha’s parinibbāna, no new maggaphala attainments occur, and the Dhamma takes on more mundane meanings.
However, if a Pragñāvantha Bodhisattva is born during a time when the human lifespan is short, not all of the 20 incalculable beings will be able to reach enlightenment during that Buddha’s lifetime. To support the remainder, that Buddha establishes the Vinaya as a protective framework—so that those who could not attain maggaphala during his lifetime may still do so afterward.
I think the videos also seem to imply that the majority of beings during this Buddhantara have already achieved maggaphala, because the “path is short” for a Pragñāvantha Bodhisattva. So, it appears that for us, the time it takes to attain maggaphala is shorter compared to other Buddha Sāsanas.
Also, I would like to add that all Buddhas are equal in terms of their Buddhahood.
The links to the videos (both in Sinhala):
Video 1: https://waharaka.com/listen/CD026-22
Video 2: https://www.youtube.com/watch?v=PX9Mv3LBMqA&list=PLYgmOkzzAFGwPWd4LrJ2TVGbo59OfSAiE&index=51