Cooldown is a statistic that was introduced in Diablo II, although it was referred to as Casting Delay. A skill that makes a drastic effect on the battlefield, such as Werewolf or Werebear, in addition to the costs normally present, needs a minor time allotment to cool down. This was set in place to prevent heroes from completely dominating the battleground. For the skills affected by this change, they were allowed buffs to their effects, justifying this minor time cooldown.
Cooldown returns in Diablo III, and it is a limit of how often a particular skill can be used. While most skills have no cooldown or have just 6 to 30 seconds, the most powerful skills may go up to 60, 90, 120, or even 180 seconds. Cooldowns longer than that are very rare.
After the skill is used, it may have no cooldown at all (the speed at which it can be used is only limited by character's or monster's resources and attack speed) or require a specific amount of time to recharge before it can be used again. This time can be visually indicated by a grayed area on the skill icon, which decreases clockwise as the cooldown nears its end. When cooldown is reset or expires, the button briefly flashes to clearly indicate that moment.
Cooldown management is a vital part of gameplay, although some cooldown-independent builds exist as well. There are many ways in which it can (and should) be reduced:
- Plain passive skills offering cooldown reduction of all skills in percent. Note that they stack with other modifiers multiplicatively, being added to the respective stat. Example: Evocation.
- Plain passive skills offering cooldown reduction of specific skills in set amount of seconds. Example: Boon of Bul-Kathos.
- Skill Runes and Items that remove, reduce or even add cooldown to specific skills. Example: Archon rune "Pure Power", Aether Walker (removes cooldown of Teleport and replaces it with Arcane Power cost).
- Respective affix on items, can roll on gloves, amulets, rings, pauldrons, off-hand items (up to 8%) or weapons (up to 10%). Some Legendary Items may reduce cooldowns of particular skills even further, like Gesture of Orpheus, reducing cooldown of Slow Time by 30-40%.
- Legendary Follower items that halve the Followers' skills cooldowns: Vadim's Surge, Slipka's Letter Opener, Hillenbrand's Training Sword.
- Items, runes and skills that reduce the remaining cooldowns by X seconds when triggered. Example: Grave Injustice, Rally, In-Geom, Obsidian Ring of the Zodiac, Trag'Oul Coils.
- Passive effects that reset the existing cooldowns, instantly removing them regardless of how much time remains. Example: Illusionist.
- Empowered Shrine and Channeling Pylon reduce all cooldowns by half and by three quarters respectively for the duration. Pylon also resets the cooldowns that are already active.
Cooldown reduction is subject to diminishing returns. There are some builds focusing on keeping lower cooldowns for long-recharging spells, but the legitimately achievable cap for most classes is somewhere around 72-75% (84.5-88.5% with Leoric's Crown).
Some skills (like Unstable Anomaly) do not have a normal cooldown, instead applying a debuff visible on the panel. These cooldowns may never be reduced, unless specifically stated otherwise.
Note that cooldown time is determined at the moment skill is used. Equipping items that reduce cooldowns by X% after the skill is used will have no effect.
Cooldowns of skills triggered by items are usually separate from cooldowns of identical skills wielded by the player, if any.
Cooldowns normally start immediately after the skill is activated, not once its effect is over. This includes shapeshifting abilities like Archon. However, some skills, especially those that grant immunity to damage, such as Serenity, Smoke Screen or Spirit Walk, will only start cooldown after they end, to prevent characters from permanent immunities.
Also note that cooldowns continue to recharge even while the character stays dead. It is possible by legitimate means to reduce a cooldown to zero before the skill effect fades. The passive skill Critical Mass for the Wizards proved so effective for that purpose that it has been removed altogether from the game.
Cooldown reduction does not go into the damage sheet statistic, but most of the time can determine the effective damage per second.
Cooldowns of item special effects (such as Firebird's Finery set bonus) are not affected by the cooldown reduction, unless specifically stated otherwise. Moreover, cooldowns of all passive skills that prevent a hero from dying (Unstable Anomaly, Near Death Experience, Spirit Vessel etc) cannot be reduced by any means possible.
Skills with charges, like Dashing Strike, actually do recharge faster, benefiting from the cooldown reduction as well.