Here is a run-down on the ATB of different Final Fantasy games.

4: The earliest implementation of the ATB system. Time pauses during animations, and there are no ATB bars. I think this version of the ATB, at least in some (2D) versions of the game, might not even have a queue; the game just looks through the battle participants to chose someone who's ready to act. I believe the character in the middle spot of the party gets priority, for example.

4 remake (3D): Time pauses during ally attacks, but not enemy attacks.

5: ATB meters are visible. Everybody's ATB fills up at the same rate, but the amount that needs to fill after an action depends on speed (unlike other implementations where everyone's ATB empties after each action and the fill rate depends on speed). Starting ATB is nearly deterministic, so you can easily manipulate turn order between your party members by manipulating speed. Time stops during attack animations. One unique feature: there's a pause whenever the command window comes up, to give you a moment to enter a command before enemies have a chance to act; this pause is affected by the battle speed setting (and is the only effect of that setting).

6: Time does not pause, even during animations, though things like regen effects are delayed until the end of the animations. This applies even on Wait mode, where time will pause in sub-menus, but actions already queued will still get to execute.

7: This is the game with three options. Active is like FF6, and Recommended like FF6's Wait, except that time will pause during (the overly long) summon animations. (One annoyance: During summons, you can't confirm any action that requires a target.) On Wait, time does pause during attack animations. (It's been a while since I've played this, and I have no desire to go back for reasons that are outside the scope of this post.)

8: Time pauses during all attack animations. I note that there's a passive ability that adds a mechanic where you can power up your summons by mashing a button during part of the attack animation (but pressing it at the wrong time makes you lose the boost).

9: Like FF6, except that regen continues during attack animations, which can get ridiculous at times. (Character with Auto-Regen low on health? Just cast a summon with a long animation and the character will be back to full by the end of the animation.)

10: Is turn-based, except for some overdrives and some mini-games that don't belong.

10-2: Time doesn't pause, but unlike the other examples here, I believe multiple attacks can be going on at the same time.

12: I believe time doesn't pause, and simultaneous normal attacks can happen, but many effects are one-at-a-time, and there's a queue they have to run through (so you might not be able to heal until another spell's animation finishes, for example).

Edit: It's also worth noting that, in some FF games (5 and 6 at least), the battle speed acts as a difficulty setting, as it only affects one side of the battle.