Casting speed
From WoWWiki
Casting speed is the time it takes for a spell to take effect. Some spells are "instant" and take effect immediately, while some spells are "channeled" and have their effects occur over the duration of the cast. Others take effect after a specific "cast time", with typically more powerful spells having longer cast times. Regardless of the type of spell, there is a global cooldown that prevents rapidfire spellcasting even with "instant" spells.
Various factors can reduce a spell's "cast time" from its base "cast time" (specified in the spell):
- Class talents: Most classes have talents that can reduce the cast time of:
- A specific spell, such as Light's Grace and its effect on Holy Light
- A class of spell, such as Divine Fury
- A school of spell, such as all holy spells
- All spells in general, such as Martyrdom and the Focused Casting effect.
- Spells: Some classes have spells that temporarily reduce the cast time of subsequent spells, such as Nature's Swiftness. Spell haste is calculated based on modified casting speed; for example, with the warlock's Bane talent, the spell haste would be calculated with 2.5 second Shadowbolts rather than 3 seconds.
- Certain items: Provide a "spell haste" bonus or a "spell haste rating" bonus. Spell haste rating is used to determine the characters spell haste bonus. At level 70, 15.7 (as of 2.2) points of spell haste rating grants 1% spell haste bonus. As of 2.4.0 spell haste also influences spellcasting global cooldown.
Certain debuffs, such as Curse of Tongues or Mind-numbing Poison) can decrease your casting speed.
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Notes
Because of the general global cooldown on spells of 1.5 secs, spell haste effectively had a cap on it for casters. This has changed with patch 2.4 — spell haste also reduces the global cooldown for spells, to a minimum of 1 second. However, spell haste still reduces the actual time spent casting, which can be advantageous when dealing with spell push back or trying to avoid interrupts.[1]
Spell haste also reduces the amount of time before the first cast takes effect, which is advantageous in the opening of duels or in situations calling for sporadic, but quick, healing.
Channeled spells are affected by haste and will see a reduction in their time spent channeling the spell, but no change in overall damage, resulting in a DPS boost. Additionally, haste scales at the same rate for channeled spells and spells with a standard cast time.
Casting speed calculation
% Spell Haste = (Spell Haste Rating / 15.7)
New Casting Time = (Base Casting Time)/(1 + (% Spell Haste / 100))
Note that "base casting time" is after talents. For example, a warlock's shadow bolt is 3.0 "base cast time" under normal usage of the term "base cast time". However, for the purpose of this formula, you should use the improved cast time from your talents (2.5 cast time, in this example) if you have that talent, or the formula will not give the correct result.
Essentially, 1% haste means allows a player to cast 1 additional spell in the time it would normally take to cast 100 spells. If the spell is a 3 second cast, that means one can theoretically cast 101 spells in 300 seconds, or 2.97 seconds per spell.
To determine the amount of time taken to cast a given sequence of spells, the reverse formula can be used, where Base Casting Time is the sum of all the spells in the given sequence:
Spell Haste Rating = ((Base Cast Time / New Cast Time) - 1) * 15.7 * 100
Example: I want my Fireball (3s), Fireball (3s), Scorch (1.5s) cast chain to take 6.5 seconds. Therefore, a [1570 * ((7.5 / 6.5) - 1) = ~242 Spell Haste Rating] is required.
Note: Blizzard rounds to 4 significant digits.
NOTE: The 15.7 and/or 1570 used in the above maths is for level 70. These numbers need to be updated for level 80.
Improving casting speed
Various sources that improve casting speed are listed on the Casting Speed improving items page.
Note: Haste rating stacks additively with itself but haste stacks multiplicatively. That means that if you have 157 haste rating, you will have +10% haste, no matter how many sources and items that haste rating comes from. If you then use troll Berserking for +30% haste, you would have 110% * 130% = 143% haste.
Losing casting speed
Chance of Ignoring Spell Interruption When Damaged = 100% - Chance to Be Interrupted By Damage
The chance to have your spells delayed by damage is additive. If you have a talent for 70% chance to avoid spell interrupts, and a paladin with concentration aura (another 35%) is in your party, you will not receive any spell interrupts from damage.
Global Cooldown
It would require 785 Spell Haste Rating to bring the Global Cooldown to 1 second.
- Percentage Reduction = (1 - (Base Cast Time / New Cast Time)) / 100
- .50 = (1 - (1.5 / 1)) / 100
- 1 = 1.5 / (1 + (.50))
Spell Haste Rating = 1570 * ((Base Cast Time / New Cast Time) - 1)
Spell Haste Rating . Base Cast Time . New Cast Time 785 1.5 1 1570 2 1 3140 3 1
As you can tell by the chart above, spell haste is not capped at 100%. Spell haste greater than 100% can be seen by mages when spellstealing the Haste buff from 
Amani'shi Flame Caster.
