From 50 fps to 11 fps in GW2

Hello,
I use the PC configuration from my signature. The only game I play (and will play) is GW2. Now, in town / dungeons / etc, with medium / high details I have close to 50 fps.

when I go to World vs World for PvP (a place where many players can be found, often in big numbers and all using flashy spells) my frame rates drops to 10-11 fps, and makes the game unplayable.

I would like to know, if possible, what causes this drastic loss of fps: is it the old video card, is it the i3 or is it a combination of something?

Thank you.