Sometimes strange things happen.. like after enabling Windows Defender in a VM instead of other Anti-Virus and after that using a VPN Client (e.g. Pulse Secure) suddenly impacting file copy on network shares.
This can be a big pain to troubleshoot. My first thought was Windows Defender doing something on the VPN or network. Because the speed was OK without VPN:
The Solution can be very simple:
Do the following in Powershell:
Disable-NetAdapterRsc -name “*”
Do the test again! Chances are great this will be the solution 🙂