GW2 is currently CPU bound, what this means is that your graphics card is doing kitten all.
At the moment, people with low-spec PC’s will experience better framerates than people with top-of-the-line gaming rigs.
Something that Anet has suggested, and I’ve seen some people get results from, is stop any overclocking you have going on either your graphics card or you CPU. I’m not entirely sure why this would help, but it does.