Hi,
Above all, as the operation system was upgraded, please check if there is a newer driver for the network adapter.
Here is an article about tuning network performance.
Performance Tuning Guidelines for Windows Server 2012 R2
https://msdn.microsoft.com/en-us/library/Dn529133.aspx
This article is included:
Tuning a Network Adapter for Network Subsystem Performance
https://msdn.microsoft.com/en-us/library/dn567662.aspx
You can try the suggestions provided in that article for tuning network performance.
Speficially you can test to enable/disable TCP Chimney Offload you can run following command lines:
netsh int tcp set global chimney=enabled
netsh int tcp set global chimney=disabled
Also we published a thread regarding troubleshooting network performance issue. Please see if it could help as well:
[Forum FAQ] Troubleshooting Network File Copy Slowness
https://social.technet.microsoft.com/Forums/windowsserver/en-US/7bd9978c-69b4-42bf-90cd-fc7541ccb663/forum-faq-troubleshooting-network-file-copy-slowness?forum=winserverPN