It's kind of misleading, because the game 'pauses' that timer so often (whenever ATB stops)
A better/more useful ballpark gauge is going by turns - breaks (15s) last roughly 2 turns normally, or 3 turns with Boon or Lunatic High's AOE haste.
Speaking of the speed setting, there's a lot of other shenanigans with it because of how the game is coded. For example, when an enemy that's not the last enemy drops loot, ATB doesn't pause, so different speeds will charge the ATB gauge different amounts during that duration of the loot drop animation.
At speed 5 it's possible (and quite common) with Autoattack on, for an early character like Cloud to get a first attack, 1-shot a mob, fill his ATB up while the mob drops loot and then get a 2nd attack off before anyone else.
Also at speed 5 every time that happens you take 3 ticks of Sap (from Self-Sacrifice, etc) which is hilariously stupid.