Having the same issues, sitting on an i5 3670k @ 3.40 GHz, and a GTX 670 with a butt ton of RAM.
Managing a clean 20 FPS in Lion’s Arch, turning absolutely cripples the FPS to 1.
Have tried tweaking every setting in both Nvidia control panel and GW2. Task Manager has nothing consuming any CPU power besides GW2 at about 40% or so.