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I have a mysterious phenomenon on a remote SQL Server 2005 which I manage and query using SQL Server Management Studio. I connect to the server through a VPN tunnel (provided by an ISP, about 8 MBit/s up and down).

When I open a new query window (first connection) the result is quickly transferred to the client. Let's say < 1 sec. When I repeat the query a few times, the results transfers are getting slower, up to 5 sec and more. It has nothing to do with the query execution time, it's just the transfer time to the client (SSMS).

Now, even stranger: I opened a second query window to the same server/database, the query result transfer is always fast even if I run them at the same time. Even after two hours the results are always transfered very quickly in that other window, while it takes much longer in the first query tab.

I'm using this testing query in order to avoid some potential caching issues:

select top 2000 newid() from <SOMETABLE>

I tried to change the server registration's connection packet size, but it didn't change much.

What could cause the problem?

PS: I posted this question on DBA SE yesterday. The people there suggested cross-posting it here, because it's not clear if it is a network database related issue.

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Just keep me posted (via if you like) if this question ends up being a network issue or a no-resolve. I'll leave yours open for now on dba. – jcolebrand Aug 18 '11 at 15:14
up vote 0 down vote accepted

For the record: I could resolve the issue. It was a network-related problem.

Our ISP found significant packet loss on VPN connections which affected IPsec decryption. They did some optimization, Cisco router updates and a reboot of the microwave radio relay that we're using.

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