The GCD is the cooldown which starts every time you start to cast a spell. At 0 haste and excluding talents(Amplify Curse) that is 1.5 seconds.
How does haste work?
Haste does two different things:
- reduces the GCD
- reduces the cast time of spells and abilities as well as reducing (some) DoT's duration by reducing the time between individual ticks(and therefore it's total duration)
The first part of this can be hard capped. What that means is that at 1269 haste you will have the GCD lowered at 1 second and it cannot go any lower that. Meaning that if you get say... 2500 haste the GCD will still be 1 second.
The second part is tricky.
First of all let's get a few things out of the way. To "haste cap" a spell means to lower it's cast time to(or bellow) the GCD. That's easier for some spells under certain circumstance then it is for others.
But why is that important?
Well... if you get a cast time under the GCD that means you will have to wait to use another spell until the GCD finishes. You will "hit" the GCD if you want to use another spell. That's what happens with instants - you press it, you wait the GCD and then you use another one.
And why is that bad?
That's bad just like having more then 17% hit is bad. If most of the spells you are using go bellow the GCD(cast time is lower then 1 second) then, from a DPS PoV, any haste that takes your spell casts under the GCD is worth 0. Just like any hit above 17% is worth 0.
So how does that affect us locks?!?
It doesn't! We don't have a haste cap. Most of our spells have long cast times(Shadow Bolt, SoulFire) or long duration(Corruption). That means that trying to get them to the point where they "hit" the GCD is almost impossible.
Like Gherkin said, you need about 4000 haste rating from gear to make a 2.25 second cast into a 1 second one. And you can't get that.
But we do have some short casts
Yes! We have UA, Immolate, Incinerate under Backdraft etc. Those spells are the only ones in any danger of "hiting" the GCD. But as far as gameplay goes, you use those spells much too scarcely to worry about them. At high enough haste levels you will hit the brick wall that the GCD is but since you do it only 4-5 times a minute that's acceptable.
What about Destro? They have Immolate and Backdraft!
Yeah, they do! Destro is easier to cap then our other specs. It's so easy that it actually has a cap they can reach - about 1300 haste. That's because under Backdraft Incinerate gets to a 1 second cast time. Any more haste then that and Incinerate's cast time will go bellow the GCD making you wait until you can fire another one. That's bad because, as i said, that haste is worthless and could(should imo) be replaced with crit(for example, or spell power) to give you more DPS.
Hope that helpes in understanding haste and it's effects. If i made any mistakes please excuse me. It's 6AM and i'm still having my cofee(god damn early morning meetings
) Correct me as you see fit!