I'm sure it was mentioned somewhere on the forums before. If you're having problems in Dalaran, turn Magic Effects all the way down in your Video Settings. For some reason many items and architectural features in this city are made to glow and have an abundant amount of magical effects going on. Even in an empty room with no other players.... Turning those down makes a huge different for a lot of folks. When someone in the guild mentions they are having lag in town, that's the first thing we always recommend and it does the trick.