They're combining two different systems antithetical to one another. One that's twitch based opposed to a more tactical and measured approach.
They are demanding attention be paid to attacking, evading, and blocking, identifying and attacking weak spots, what each enemy is doing at any given time, paying attention to HP/MP management of not only yourself but that of your allies, and menu management and respective skill utilization that can vary for enemy to enemy. It's a lot to place on the player's plate under pressure, and yes, the game pauses to consider this when ATBs fill and the command menu is called up, which just goes to show to me how incompatible these system they've tried to combine really are. When the devs shoulder me with action game considerations in the heat of the moment in battles with many enemies and variables to consider, it can be trying to keep track of them all to their most effective and efficient implementations. Is it workable? Yes, but I don't think it's enjoyable, nor lends itself to the fullest potential of what adhering to a true turn-based system could afford.
Classic mitigates the aforementioned somewhat by lifting the superficial actiony nature of the game out of the player's hands which I appreciate, but there's still issues apparent after some play put in. Not to mention the default to easy difficulty, the AI leaves something greatly to be desired not only in how it conducts the player's character in a dynamic situation, but especially that of the NPCs whose ATBs take forever to fill unless you switch to them and take over......which defeats the entire purpose of being in Classic to begin with. What they should've done if they insisted upon this direction was to allow the player some nuance and micromanagement in how the AI controlled NPCs during battle, and re-balanced the ATB so that equal opportunity could be afforded to each character regardless of who is hammering the attack button at any given moment.
To be perfectly honest, this is one of the poorest battle systems I've seen in a game. It's unbalanced, it's chaotic, and I always feel like every time I triumph, it's never due to a measured or skillful execution of tactics on my part but one of brute forcing my way through it, and one further that I could've accomplished alone, aided by potions and spells.