Haste is a hidden stat in Final Fantasy XI. No job has an innate Haste% above 0%; it can be raised by infusing certain Atma while in Abyssea, infusing certain Atmacite while engaged in Voidwatch, or – most commonly – by equipping equipment that has a +Haste% stat. The Haste, Flurry, and March status enhancements then add to the Haste stat.
The effect of the Haste stat is to reduce delays on melee attack rounds and spell recast by the appropriate amount. Thus a total of 50% Haste reduces delay to half of normal, which doubles attacking speed. A total of 67% Haste reduces delay to 33% (one third) of normal, which triples attacking speed. 75% total haste reduces delay to 25% (one quarter) of normal, for quadruple speed attack. Thus, unlike nearly every other stat in the game, increasing Haste offers increasing returns. The more Haste you have, the more you gain from increasing it further.
In order to prevent players from focusing solely on +Haste% gear, as well as to prevent players from eventually reaching infinite attack speed and breaking the game, Square Enix has implemented several caps on Haste. There is a total cap on all forms of attack delay reduction of 80%. Furthermore, there are three categories of Haste, based on source, each with its own cap within that category. "Equipment haste" caps at 25%. "Job ability haste" also caps at 25%. "Magic haste" caps at 43.75%.
Incidentally, although Haste is always written as a percentage in-game, it actually is calculated internally in units of 1/1024 rather than 1/100 – using fixed-point binary arithmetic for efficiency. Because 100 does not divide evenly into 1024, what that means is that the Haste% you see on equipment isn't perfectly accurate. The actual amount of Haste you gain from most equipment is ever so slightly less than what is listed in the equipment stat window, so truly knowledgeable players know that you need 26% worth of equipment haste to be capped.