Alimax, check
this webpage. Applies to Windows 2000, but who knows ...
The article talks about 'Security update 893066 returns the default size of the TCP receive window in Windows 2000 to the pre-SP3 default size of 17,520 bytes', and it says:
After you apply security update 893066, you may notice that the performance of the network decreases. For example, the throughput may be one-fourth of the throughput that you experienced before you applied the security update.
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Edit: Ouch, i didn't notice that you've already checked that page
.