Half the speed migrating from Windows 2000 to Windows 2003

A long time ago I had written a very simple server that uses AcceptEx,
TransmitFile and IOCPs. It works fine on all versions of NT. Today,
out of curiosity, I ran the server on a virgin Windows 2003 Server
(with sp2) machine and was surprised to see that its performance was
less than half that of the same machine running virgin Windows 2000
Server (with sp4). On w2k, my server gives a speed of ~6700 responses
per second (rps) as opposed to ~3000 rps on the W2k3 machine.

The test configuration:
1.6 G Pentium, 1G RAM. 1000BT ethernet.
Each request/response pair is about 100 bytes each. The socket is
closed after each response.

I have not been able to figure out what could be the reason why such a
drastic drop in performance from an OS that is marketed as given
better performance than its predecessor. I would be grateful if
someone knew the answer offhand.
.

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