The first thing you'll need is a casting bar addon called "Quartz"
You can find this mod at: Quartz - WowAce Wiki
What Quartz does that's different than other casting bar mods is that it looks at your latency and then marks your cast bar accordingly with a "red" section in it, according to how high your latency is. The higher your latency, the longer the red part will be. The bar still shows the full cast time, but there will now be a varying amount of red on the bar.

So, how is this going to help me do more DPS?
Well, Blizzard puts a buffer on the cast bar to attempt to equalize the advantage of someone with good latency vs someone with bad latency. While the server takes a variable amount of time to register that your spell has been cast, your client (the cast bar you see), is a flat amount of time.
Usually, you cannot begin casting your next spell until the casting bar has reached its end, but with a certain macro, as well as Quartz to show you when to use it, you're able to begin casting your next spell as soon as you reach the buffer time on your cast bar that Blizzard puts there for latency, effectively reducing your casting time relative to your latency.
So how do I do this?
You will be using a stopcasting macro, which looks something like this:
Code:
/stopcasting
/cast (your spell's name)
Code:
example
/stopcasting
/cast Shadowbolt
Replace your spells with a macro that has this code in it.
Now, the problem with this is the inability to spam the button. When you hit the button, it will stop whatever cast you are currently casting and it will begin to cast the next spell. If you hit it too early, it will cancel your current cast! So get ready to stop spamming your buttons.
So, when you cast your spell, and the casting bar reaches the red part with the small latency text, cast your next spell (as long as that one has also been outfitted with the the 'stopcasting' macro), and watch as your time spent casting decreases by about 10-20%, and your DPS DIRECTLY increases by about 10-20%!
Blizzard implemented the client side increased cast time so people with higher latency wouldn't get screwed compared to faster internet people.
Using this macro/addon gives you a gameplay advantaged and will possibly become a bannable offense, or at least patched in some way.
Maybe an exploit, maybe a guide, I dunno - But i posted it in exploit section
It was posted some time ago by paperboi here but it was hard to follow.
thank you snarg