Final Fantasy XII, like most Final Fantasy games, provides you with an option to change the Battle Speed within the configuration menu. Quite often inexperienced players will lower the Battle Speed in order to make the game easier. Even more experienced players will quite often select a speed somewhere in the middle to allow them enough time to employ strategies during each of the fights. It is important to note the effects that the Battle Speed option has in this game specifically.
Most Final Fantasy games used a “wait time” mechanic which requires each player to wait a specific amount of time before they can use a move. Final Fantasy XII uses a different mechanic; rather than a wait time, each character’s move has a charge time associated with it. Battle Speed directly affects how quickly (or slowly) the “charge time” meter fills.
Changing the Battle Speed does not affect movement speed on the battle field, or the amount of actual time that a beneficial status buff (like Protect, Haste, Bubble, etc.) stays active on party member.
Generally speaking, this means that reducing the Battle Speed to “slow” will have a negative affect on the player. In fact, during some fights, a slower Battle Speed can significantly increase the difficulty of the battle (such as Yiazmat). As such, it is recommended that you keep the Battle Speed near the Fast setting if possible.