File transfer over the network one day becomes very slow.

I've recently had this very interesting problem with a Windows 7 Pro 64bit workstation that suddenly had a very low speed when transferring files over the LAN. What happened was the speed of listing files dropped about 10-20 times the normal speed. The actual copying of the files - once initiated - was normal (about 90% of the 100mbps connection).

I started diagnosing and optimizing the network speed to no avail.

What didn't help:

1. Removing Remote Differential Compression from windows components.

2. Disable Autotuning by running the command:

netsh interface tcp set global autotuning=disabled

3. Removing IPv6 from Network Properties.

4. Clearing DNS cache.

5. Resetting the TCP/IP stack by running:

netsh int ip reset reset.log

6. Resetting Winsock by running:

netsh winsock reset catalog

7. Tweaking the registry based on this article on MSDN.

Finally I decided to check the NIC - which was Intel 82579V Gigabit Ethernet Card.

I disabled the Large Send Offload v2 for both IPv4 and IPv6. Didn't help.

The solution was quite simple (as I suspected).

Something went wrong with the NIC driver. It wasn't an automatic update from Microsoft either. This driver was never updated.

I decided to run the built in diagnostics on the driver window. Everything came back normal.

But when I switched the Link speed to 10mbps - immediately there was an improvement. But of course - we're limited to only 10mbps. So I switched back to 100mbps (auto) - back to the old speed problem.

I went to the Intel download center and got the latest driver for this card (4 day old!) After installing it - everything went back to normal, even better because of the small improvements I made trying to fix the problem.
So this shows that when copying or downloading files over the network takes too long (only on one particular station in the network) - it is best to start from setting the link & duplex speed of the NIC to 10mbps to check for improvement.