Vlad Drumea troubleshoots a pernicious wait type:
In this post I’ll go over what the ASYNC_NETWORK_IO wait type is, when it occurs, and how you can tell if the application is causing it.
Lately I’ve had to troubleshoot a few situations where the issue seemed like a poor performing query, but the apparent slowdown was due to applications causing excessive ASYNC_NETWORK_IO waits in SQL Server.
This particular wait doesn’t always happen because of slow apps, though that’s often the case. Vlad breaks out several reasons why you might see the wait and explains what you can do to fix the problem.