I felt that I have to post this so nobody will be misinformed:
Guild Wars 2 uses only 1 core. Which means, that users with i7 will have max 12,5% load because i7 has got 8 cores (4, but Hyper-Threading doubles it). If U have 20-25% then it means that U have 4 cores. Etc etc.
Get SpeedFan and see it by yourself Or just use task manager and see the lines in performance.
We must hope that ArenaNet will update the game to use more cores and rely more on the GPU, because for now U can have 8800GTS or 560Ti it seems to doesn't matter.
Keep in mind that this is a beta test (one of many) and beta should never be taken into account in deciding "if the product is worth my money?".
U can say: oh, Diablo 3 open beta was fluid and flawless.
I say: yes, but Diablo 3 is almost finished, while GW2 is like 2/3 finished. They just need to work on the performance, besides of that, GW2 is amazing! <3 Happy playing!