Guild Wars 2 beta is getting hammered. No, it's not slipping off to the bar to order shots until it can't drive home, but it is getting filled with people eager to try out the game. To compensate, ArenaNet has just announced that world population limits are being increased for the beta event, meaning that several worlds previously listed as being full are now open to more characters. This may not fix the issue, but it should help alleviate some of the stress from the rush.
Of course, the game is still in beta, and that means that not everything in the game is fully polished. Want to make sure that there's actually a problem and not just an issue with your computer? Check the updated known issues thread, which has an extensive set of understood issues. Perhaps most globally relevant is the fact that the game is apparently very heavily bottlenecked by the CPU; if you're having performance issues, lowering the video settings may not help you for this round of testing.