Use Report Lag tool for latency.

Q u o t e:
just started, very bad lag, been casting for about 5 minutes now, cant log out, cant check mail, auction house, anything. just reporting

Actually, there's no need to keep a thread here bumped, guys. While we appreciate the heads up, you may find that using the Report Lag tool is your most direct line to express concerns regarding unusual levels of realm latency.

Here's a guide on how to use it:

http://us.blizzard.com/support/article.xml?locale=en_US&articleId=30247

Q u o t e:


Hmmm I don't see instructions on how to report lag when I am stuck for 10 minutes at a loading screen :(

That's because that isn't latency, but an entirely different issue.