I was having AWFUL performance with my GTX 670. Even with supersampling and everything cranked all the way up it would not force my GPU to work. I would get 30-50% GPU usage and 20-30 FPS.
I fixed it by going into the NVIDIA control panel and selecting “Prefer maximum performance” for Guild Wars 2 under power management. Now I get 80-99% GPU usage and 45-60 FPS with all the settings cranked. Hope this helps some of you guys.