When I launched GW2 just now it said I'm using a graphics driver with known issues. Since when? I have the latest driver straight from the nvidia site (301.42 for my 580) :/
You should update to the 304 beta drivers, I've seen people say it has increased their GW2 performance by ~ 15-20 on avg. Not saying you need it but hey it can't hurt. See if that also gets rid of the issue?