Haste Rating is the amount of points you accumulate to achieve Spell Haste. Spell Haste determines the speed at which you cast your spells and NOT a reduction to the base cast time of your spells. It pretty much means how fast you can cast your spells. Latency affects your casting speed.
Being hit while casting a spell will increase the time to cast the spell by 0.5sec per hit, to a maximum of 1.0sec per attack.
True Haste Rating per 1% Spell Haste;
Casting time calculation, Haste Rating as a variable;
Code:
% Spell Haste = (Haste Rating / 32.79)
New Casting Time = Base Casting Time / (1 + (% Spell Haste / 100))
Here is an example, using my Mage who has 1,050 Haste Rating
(Empower Frostbolt & Improved Frostbolt reduces Base Casting Speed of Frostbolt by 0.7sec to 2.3sec;
Code:
32.02% Spell Haste = (1050 / 32.79)
1.74sec = 2.3sec / (1 + (32.02% / 100))
That means that with 1,050 Haste Rating I cast 32.02% faster, and my Frostbolt takes 1.74sec to cast. Now let's figure out how much Haste Rating we need to make me cast my Frostbolt at 1.73sec.
Desired casting speed, and how much Haste Rating is needed for it;
Code:
Haste Rating needed = ((Base Cast Time / Desired Cast Time) - 1) * 32.79 * 100
My example using the same previous variables;
Code:
1,080 = ((2.3sec / 1.73sec) - 1) * 32.79 * 100
So I will need 30 more Haste Rating to be able to cast my Frostbolt at 1.73sec
Haste Rating also lowers your Global Cooldown (GCD) to a minimum of 1.0sec(50% casting speed) from the base GCD of 1.5sec (if you have no Haste). Your GCD is the time between your next spells, when you begin casting your spells they go into a cooldown which prevents you from casting anything with a GCD until it is over. It is also important to know that your latency affects your GCD and spell casting speed.
Amount of Haste Chance required to obtain desired GCD, and the Haste Rating required;
Code:
HasteChance = ((1.5 / DesiredGCD) - 1) * 100
Rating = (32.79 * 100) * HasteChance
Here's an example, if I wanted my GCD to be 1.0sec;
Code:
50% = ((1.5 / 1.0) - 1) * 100
1,639.5 = (32.79 * 100) * 50%
I would need 1,640 Haste Rating to have 50% Haste Chance, which will lower my GCD to the minimum of 1.0sec
Active spells & abilities that do not have a GCD that affect your spell casting;