I got Full Fibre last year from Shell. On usenet and irrespective of what software i use, NZBGet or SabNZB, If the server is setup with Linux full speed. If setup on Windows 11 it starts off at 100% but slows very quickly to around 20-30% and stays at that.

I bought a pci express 2.5G Ethernet card thinking it might be Win 11 and the onboard ethernet that was the issue. This didn't solve the problem.

If I connect my mini pc with Win 11 and install the software 100%.

I am completely stumped as to what the issue might be with the Server and Win 11

Anything else I could try?