I've seen this argument a lot and I really hate it. I feel like people who criticize the pacing of Persona 3 are looking at it less like a video game and more like a movie or TV show. What led me to make this argument was a meme on Reddit of all places, r/okbuddypersona to be specific. The meme basically said that Persona 3 is carried by the last 5 or so hours, which I find really absurd, so I wanna discuss how I feel that Persona 3's pacing is actually pretty well-done.
SPOILERS FOR ALL VERSIONS OF PERSONA 3
The crux of this argument is that for the most part, only one major event happens each month of the game. But the thing is, the game manages a pretty delicate balance between story and gameplay.
On a typical day in Persona 3, you have two time slots, the night-time slots for the most part are reserved for Tartarus trips and the day-time is only used for social links. (This works differently in Reload but I'll get to that in a later section). So even while story events aren't happening, it is hard to notice since you are still prety busy with other events in-game. Persona 3's story therefore can let plot-points drip down and not lose the audience.
Now you might be wondering: that doesn't seem like a lot of stuff to do, well...
Both Reload and the OG/FES handle the lack of time very differently. You may disagree about whether or not it works for you, but in my opinion both of them work with the dead air pretty well.