Haste

Haste increases attack speed and casting speed.

Haste stacks in a multiplicative manner. This means that it is beneficial to stack multiple haste effects. For instance, stacking Slice and Dice with Blade Flurry gives a total of 68% haste (140% * 120% = 168% of the base attack speed). Haste rating stacks additively with itself (two sources of 100 haste rating give 200 haste rating stacked together) and then is converted into a percentage that stacks in a multiplicative manner with other sources of haste. For every 32.79 points of haste rating you will have 1% faster attack speed at level 80.

Casting time and the auto-attack interval are calculated by dividing the weapon or ability's base attack or casting time by the attack speed or casting speed. Thus, 100% haste cuts casting time in half.

Haste cap
Casting speed cannot reduce the global cooldown below 1 second. Thus, once you have enough haste that all the abilities you use in combat have a casting time of 1 second or less, additional haste rating won't increase your damage per second or healing per second from abilities (even on fights that are too short for mana to become the limiting factor). For casters, this can be seen as a "haste cap". For non-casters, however, DPS continues to increase linearly beyond this point -- albeit more slowly -- because auto-attack speed increases with no limit.

[[File:Cataclysm-Logo-Small.PNG]] Cataclysm
In World of Warcraft: Cataclysm, haste will also increase the rate at which you gain energy, runes, and focus. Retribution paladins and Enhancement shaman will have a talent that allows them to take advantage of this benefit.

[[Image:Wrath-Logo-Small.PNG]] Wrath of the Lich King
Early numbers are showing a 32.79 rating required at level 80 to achieve a 1% gain in speed.

Patch 3.1
Quivers and ammo bags no longer grant haste. Instead, the haste from quivers and ammo bags has been built into Auto-Shot. Some classes' haste got buffed and now 25.21 Haste Rating provide 1% melee haste for Death Knights, Druids, Shamans, and Paladins.

As of patch 3.0, Haste Rating also increases your Spell haste.

Patch 2.2.0
Haste has been rebalanced. It has returned to the ratios from the launch of Burning Crusade. Melee attacks and spell casts will now benefit at identical rates from haste. This change results in a reduction in the benefit of haste for melee attacks and an increase in the benefit for spellcasters.

Patch 2.1.0
The amount of haste granted by a point of haste rating is being reduced again. See combat rating system for current rating effects.

10 haste rating gives 1% haste at level 60.

Patch 1.12
Previously, haste and slow effects worked inconsistently, with spells working differently from weapons, and hastes and slows not acting as inverses of each other. We have revised the system so that all haste and slow effects work the same way, and haste and slow percentages of the same magnitude perfectly cancel each other out (30% haste and 30% slow combine to no change). As a result, we had to change the tooltip numbers on all spell haste effects, and on all melee and range slow effects. The numbers in the tooltips are different, but the game functionality is unchanged (other than slight rounding errors). Those tooltips that changed will now display larger numbers than they used to display. Conceptually, haste values indicate how much more of that activity you can perform in a given time. 30% melee haste means 30% more swings in a given time. Slow values indicate how much longer an activity takes to complete. 30% slow means an action takes 30% longer to finish.

Haste now stacks in a multiplicative manner.

In development
Changes are likely coming for haste in Patch 3.3.0. Here's a post from the lead developer describing some changes that might be seen: