Diablo 3 is currently going through some serious lag issues, but Blizzard is on top of it. If you are having such problems, you can help by reporting the info to the Battle.Net forums.
What you need to do is run a trace route, and also run a utility called Looking Glass. You can download Looking Glass here and get instructions on a trace route here. Once you’re done, go back to this stickied thread and share your results.