All of the core Final Fantasy games from Final Fantasy IV through Final Fantasy IX used what the call the active time battle system. This is a real time system where you input commands for your characters one at a time and everyone's turn to take an action can come up while you're thinking about what to do. Or while an animation is going off. There were tweaks as the games progressed but they were all the same basic idea, and they all really had the flaw that the enemies could take a better advantage of spell animations than the player could. You ended up needing to turn on the 'wait' option and then go into the item menu with another character while spells were animating in order to make the system work properly.
Final Fantasy X changed it all up by basically forcing time to pause for all characters while anything was happening. Spell animations, selecting your command from a menu, falling asleep in your chair... Everything is paused. It was very much a more strategic combat system, and I for one really liked it. You'd think Final Fantasy X-2 would be more of the same, but they decided that keeping the world and characters from the first game was all they needed to keep and went in a completely different direction with the battle system.
Where Final Fantasy X took time out of the equation Final Fantasy X-2 decided to go all in on real time. Not only do your action timers charge up while other things are happening, other things can happen while other things are happening! In all of the other Final Fantasy games one action happens at a time. If I'm stepping forward to swing my sword then that's what is going on right now. This is not the case in FFX-2, where multiple people can be attacking at the same time. They actually made it a thing where you do bonus damage if you chain your attacks together in a short period of time! Yuna's starting class actually has an ability that attacks ~12 times in a row so small damage, but it stacks the chain multiplier up pretty high and can make for some big bonus damage.
There are some weird things going on with the system. Auto attacks happen as soon as you enter the command (with some classes having ranged attacks so they really are instant and others being melee so you need to watch the animation of running up to swing your sword before the damage happens) but spells get an extra charge bar after you enter the command. After the charge time happens you cast the spell. You then immediately get to enter another command, which is nice, but you essentially end up paying the delay for a spell before you get the spell and the delay for an attack after you get the attack.
There's also the issue with a character being in multiple animations at once, which doesn't seem to be able to happen. This opens up the option of delaying an action by hitting the person! I recently had my party set up as three ranged attackers (gunner, gun mage, psychic) and was fighting a big robot thing that had a wind up attack for big damage. He'd start running towards someone to attack him, and I'd hit him with attack after attack after attack. Each time he got hit he'd stutter a bit, which meant he took an awfully long time to actually do his damage. He probably only got to attack a third as much as he should have. This felt good. But when the enemies get to do it to me it feels terrible. I had Rikku as a black mage for a while but this mechanic made it so she got to attack less often than she should, and she had a limited mana pool. So once again I feel obligated to take auto attack jobs in a job game.
I was hoping I was missing something so I decided to go look up a mechanic guide to see how the ATB system worked but there's not much of a mechanic guide for this game. I did find a bunch of people complaining about how terrible black mage spells were, and I found a bunch of people saying how awesome the chaining system of auto attacks is. I did find a damage formula guide which showed that physical attacks square your strength and magical attacks are linear with magic power which means spells are going to suck in the late game in FFX-2 just like they did in FFX.
Oh well... This isn't actually bad for me. I like auto attacks, after all, and tend to avoid using my limited amount of mp on spells anyway. So having spells do less damage and take longer doesn't really hurt my default plan. It's just disappointing that things aren't close to balanced. Especially since Rikku had such great puns in her black mage form...