I’m in the same boat as you my friend. My PC is not up to par, but, it is above the minimum requirements to play the game. I just got a new processor and started fresh on this PC. GW2 is the first game I installed and for about a good month and a half it played wonderfully, with low settings, getting around 45fps average. Yesterday I started up the game and my fps dropped down into the teens.
I’m not much of a PvPer I mostly just like running around and killing mobs. However, I decided to go into WvW just to check it out. That is the first thing I did different. I don’t know if the first time going into WvW if it downloads some extra feature but ever since I went to my servers borderlands, and came back to the game, my FPS has been low. The only other think I have done was downloaded another game that installed DirectX 9 which I thought was already on my PC. If you decide to re-install GW2 I would like to know if that helped with you fps issue.