Why do async_network_io wait types occur?
ASYNC_NETWORK_IO somehow indicates that the client application isn't processing results as fast as SQL Server feeds them. This could be caused by an issue with the client application or with the network connection between the server and the client application.
Please refer to a post by Thomas LaRock
The ASYNC_NETWORK_IO wait indicates that one of two scenarios are happening. The first scenario is that the session (i.e., SPID) is waiting for the client application to process the result set and send a signal back to SQL Server that it is ready to process more data. The second is that there may be a network performance issue.
or this post by Joe Sack
As you may already be aware, the ASYNC_NETWORK_IO (seen in SQL 2005) and NETWORKIO (seen in SQL 2000) wait types are associated with either a calling application that is not processing results quickly enough from SQL Server or is associated with a network performance issue.
Since you are using entity framework this post by Brent Ozar may be useful too
Looking at the wait stats for these queries, I saw there was a lot of ASYNC_NETWORK_IO — often 1000+ milliseconds. That didn’t make any sense either! How can a query with so little CPU time and so few reads take so long to complete? It’s not like the application was asking for millions of rows and couldn’t consume the results fast enough.
There are some misconceptions regarding the ASYNC_NETWORK_IO wait type, primarily because of the name that indicates the network issue, but it is quite rare the reason for this wait type.
Excessive ASYNC_NETWORK_IO waits could occur under two scenarios:
The session must wait for the client application to process the data received from SQL Server in order to send the signal to SQL Server that it can accept new data for processing. This is a common scenario that may reflect bad application design, and is the most often cause of excessive ASYNC_NETWORK_IO wait type values.
This involves investigating the application that is causing the excessive ASYNC_NETWORK_IO wait type values and often coordinating with the application developers who created it.
Network bandwidth is maxed out. A clogged Ethernet will cause the slow data transmission back and forth from the application. This, in and of itself, will degrade the efficiency of the application.
Much more details can be found on this page