I have the exact same problem. I have an i5-2410M Processor and Nvidia GeForce GT 540M video card. And a friend of mine with a worse video card and a 2GHz Intel dual-core processor runs the game without problems even in big WvW battles… so I started thinking that maybe there is a tweak or something that I must do for the game to run properly… My bigest problem is that in Beta Weekends, when I played in the WvW, even if I were in a big battle, I had no lag at all… and now , like you said, it runs with 5-10 fps… so maybe someone can tell me what may be the problem ?!
Processor: Intel Core i5-2410M
GPU: NVIDIA GeForce GT 540M (2Gb ddr3 memory, 128bits)
RAM: 4GB ddr3