No matter what your reaction time, you can't start reacting until you have the information. And once you DO react, your actions do nothing until they travel thru the internet and reach the Blizzard Server.
Never mind what speed you pay an ISP for, Who thought it was a good idea to seed a torrent, Or the fact that your router is held together with scotch tape and chewing gum...
There is a problem!
Windows computers and World of Warcraft have problems talking to each other in their natural forms. They just don't send data between themselves in a manner that makes sense.
In layman's terms, I'm at a drive-thru window, and I order something... and get no response, so I shout another thing on my list. Then the guy says, "Got it, OH and I got the first thing you ordered." and then I have to cross two things off my list.
This doesn't seem too bad until you realize it happens hundreds of thousands of times in seconds. The result of these orders sitting in limbo = LATENCY.(lag)
There is a solution!
People way nerdier than me developed a FIX for this issue.
It's called Leatrix Latency Fix; Safe and reliable for years.
The program (and a very thorough installation guide)can be found here