A Spell’s Casting Speed is the time it takes for the Spell to take effect. Some Spells are instant cast such as Ice Lance or Cone of Cold, which take effect immediately. Some Spells are channeled Spells like Blizzard, which also takes into effect immediately, and stays in effect during the entire duration as long as the spell caster remains still and not break the channeling. Other Spells take effect after the Spell’s cast time such as Frostbolt. Dispite what kind of Spell it is, there is a Global Cooldown of 1.5 seconds which prevents rapidly casting Spells quickly.

There are many factors which affects the Casting Speed and cast time of Spells. Most Classes have talents which reduces the base cast time of spells. For example, Improve Frostbolt reduces the base cast time of Frostbolt from 3 seconds to 2.5 seconds with 5 points into the talent. Also there are abilites which may temporarily reduce the cast time of subsequent spells. For example, Icy Veins increases your Casting Speed of all Spells by 20% for 20 seconds.

The last factor which affects Casting Speed is items or equipments which provide Spell Haste bonus or Spell Haste Rating. As of Patch 2.2, for a level 70 Spell Caster, 15.7 Points of Spell Haste Rating will give you 1% Spell Haste bonus. Also as of Patch 2.4, Spell Haste also reduces the Global Cooldown of the caster, to a minimum of 1 second. This removed the “Spell Haste Cap” for casters, which would only allowed casters to stack Spell Haste until their primary Spell was reduced to 1.5 seconds, which was the Global Cooldown.

Channeled Spells are affected by Spell Haste the same way as Spells with a cast time. Instant cast Spells are not affected directly by Spell Haste, but with the reduction of the Global Cooldown by Spell Haste, you may see instant cast Spells be used more frequently. For example, with enough Spell Haste, you could be able to cast an instant cast Spell every 1 second, instead of every 1.5 second.

The first thing you must understand about Spell Haste is, that it doesn’t reduce your Spell’s cast time, but it increases your Spell Casting Speed.

“What the heck does that mean?” Say you manage to get 1570 Spell Haste Rating (or 100% Spell Haste). If Spell Haste reduced casting time by 100%, it would make the Spell instant cast. But instead, Spell Haste increases your Spell Casting Speed by 100%, that would mean you cast twice as fast, which makes the Spell cast in half the time.

So Spell Haste increases Spell Casting Speed, NOT reduce Spell cast time.

Now for some number crunching!

Casting speed calculation

Speed = 1 + (Spell Haste Rating / 1570)
New casting Time = Base Casting Time / Speed

or just put them together

New Casting Time = Base Casting Time / (1 + (Spell Haste Rating / 1570))

Note that “base casting time” is after talents. For example, a Mage’s Frostbolt is 3 second “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 second cast time, in this example) if you have that talent, or the formula will not give the correct result.

Also remember Spell Haste affects your Global Cooldown now. If you want to bring the Global Cooldown down to the lowest, it would require 785 Spell Haste to bring the Global Cooldown down to 1 second.

1.5 / (1 + (785 / 1570)) = 1 second

Essentially, 1% haste means a player can cast an additional spell in the time it would normally take to cast 100 spells. If the spell is a 2.5 second cast, that means one can theoretically cast 101 spells in 250 seconds, or 2.475 seconds per spell.

Haste rating stacks additively with itself but Haste effects 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 Icy Veins for +20% haste, you would have 110% * 120% = 132% haste. NOT 110% + 120%  = 130%.

392.5 Spell Haste Rating to bring your Frostbolt down to 2 seconds.

So for me with 142 Spell Haste Rating. My Speed = 1 + (142 / 1570) = 1.09 = 109%
Now I take that and enter it into the second part of the formula:

New Casting Time = 2.5 / 109% = 2.29 seconds

If I were to cast Icy Veins with my current Haste Rating. The Haste will stack multiplicatively and be: 109% * 120% = 130.8%

New Casting Time = 2.5 / 130.8% = 1.91 seconds

If you happen to have a Shaman in your group for Heroism or Bloodlust. The Haste will be: 109% * 120% * 130% = 170%

New Casting Time = 2.5 / 170% = 1.47 seconds

Should I take Spell Haste over Spell Damage?

This is where I use one of our Useful Mage Tools from earlier. By entering our stats into Lhivera’s Theorycraft-o-Matic, we can get an idea of how much Spell Haste is worth to us. When I enter in Utada’s stats, I get:

According to this, 1 point of Spell Haste Rating is worth more than 1 point of Spell Damage, and since I am Spell Hit capped, that stat will no longer improve my DPS. 
So for me:

Spell Haste > Spell Damage > Spell Crit

Also once again, Spell Haste increases your DPS and not your DPM. If you don’t have the mana to support Spell Haste, you need to learn to manage your mana (mana pots and mana gems) or you can’t stack Spell Haste.

Sources: WoWWiki.com and ElitistJerks.com

If you find any errors or have any questions let me know. It was hard to explain some parts, so my wording may have been confusing ^^;

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