A post from Blizzard acknowledges reports of "major lag" in Diablo III, asking users for trace routes and other details to help suss out the problem. Here's word:

We've been asked to collect some information for our network engineers on this issue.
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We'd like to have your trace route info when you trace to 12.168.209.68. We'd also like to see the results when you run our Looking Glass utility to see the return path of the trace from Blizzard to you. Be sure to select Diablo III in the utility. And lastly, please identify your ISP and geographical location in your reply post with the above info.