A lot of FFIX's decisions - especially the way Trance works - are semi-sensible reactions to the way that the Limit Break systems worked in FF7/8 - Trance is set up to discourage people from the common tactic of hoarding Limit Breaks and spamming them on bosses for an effortless kill (something that was also a problem for FFX). But it ends up just being a weird feature that never quite finds its role in things.
The problem with FF9's ATB actually isn't that it's slow - though it is, and that's annoying early in the game - it's that it doesn't have a pause-timer function.
FF4 and 5 pause the timer anytime an action takes place, even normal attacks (you can see this by watching the ATB bar).
FFVI sped things up a tiny bit by not having the timer pause during regular attacks and simpler spell animations, and introduced something that's essentially an animation queue - if several inputs are bunched together, they'll just play out in sequence, not right when your command is entered.
The pause-timer function, in turn, allowed for more elaborate animations in FF7 and 8 - limit breaks and summons pause the ATB while they're playing out so they don't cause issues in the animation/turn queue.
FF9's problem is that the timer never pauses during any animation ever - so during longer battles or following any particularly long animation, you're still queuing up more turns (which further delay the animation queue, ad infinitum). This is why FF9 alone suffers from the problem of having your actual turns play out sometimes over 60 seconds after selecting an action, often when the action is no longer context-appropriate (hello, Heat status effect). This is also why auto-Regen is so overpowered in FF9: you keep regenerating every second even during immensely long summons, so simply deliberately slowing the battle by bringing in the longest animations possible will keep you at full health basically all of the time.
It's an objectively bad design decision and speeding up the ATB speed doesn't fix it.
