Recently my guild decided to switch servers because of the unbearable lag during thaddius and other frequent lag-related issues. On this server, I seem to be getting a lot less server lag, but a lot more graphical lag. I was thinking that maybe all my addons are still loading variables from other servers, which would make sense, since a different server shouldn't affect my fps.
The server your palying on shouldn't affect your lag (then again anything is possible I suppose, depends on your addons I suppose.)
If you want to test your theory, rename your WTF folder to WTFold and launch the game. You'll have a fresh set of squeaky clean settings for everything. If it dosen't seem to affect your game's preformance, trash the new WTF folder that WoW made, and rename your WTFold folder back to WTF. You'll have all your old settings back :)