I don't even.. how is this.. what the bloody...
You do realize that what you see on your "VIDEO" screen is not only drawn images of variable objects, spells, effects and scenery but also physics AND MOST IMPORTANT THE FUCKING COMBAT MECHANICS. If you own a modern graphics card it's perfectly capable of doing all the first mentioned without much hiccups.
Graphics cards are not made for heavy calculations that you need for calculating object movements and going trough 1000 different spells that everyone shoots around in a 25man raid setup, yes the client has to go trough a lot of other combat information in addition to your own. There's also networking related stuff that can be a hog when the client has to do numerous back and forth checks between the server and you.
The fucking bottom line is, when you usually experience lag, usually in an environments with a lot of people moving, the weakest part of your setup is your CPU since it can't cope with all that workload that can't be processed by the graphics card.