Increases speed temporarily
A good source of Haste is Vysage, as not only can the spell be drawn from it, but it is a good source for Lightweights, which can be refined into 20 Haste spells. Cactuars also drop Lightweights and on the Cactuar Island the player only encounters them in large quantities, making amassing Lightweights easy as long as the player is fast enough to kill them before they flee.
Haste applies the status effect of the same name, which increases the target's speed for 32 seconds. It is cast using the Magic command or Selphie's Slot Limit Break. Casting Haste in battle increases compatibility with Carbuncle by 1 and with Cerberus by 0.6, but lowers compatibility with Eden by 0.2.
Cerberus teaches the support ability Auto-Haste, and Accelerators can be used to teach the ability to any Guardian Force. Auto-Haste renders casting Haste redundant, as it is permanent and cannot be removed. Ifrit also teaches Mad Rush, a command ability that casts Haste along with Berserk, making it riskier.
Haste provides decent stat boosts, especially to Speed, for which it provides the third-highest boost. As the player can obtain the Auto-Haste support ability, it is better to junction Haste to speed rather than cast it.