I too have been hit by a sudden and unexpected lagfest. I can run around Wvw maps like Tom Cruise in Times Square in Vanilla Sky and BOOM! I am suddenly a splat on the floor.
I tried this: (WARNING! I am techno useless and have no idea if it is detrimental to your hardware, and is advisable. Do it at your own risk. In fact, I would prefer it if someone more PC savvy gave it the nod before you tried it.)
Once GW2 is running, control-alt-del to the task manager. Find the GW2 process. Right click it. Set priority to high.
It improved performance no end for me. Your mileage may vary.