You cannot use NFS shares over windows as the NFS client in Windows runs only in userspace, and maps the NFS volumes only in user sessions. So there's no way for the user that runs in unattended mode the Veeam service to do the same mount. NFS mount are not persistent over user sessions as they are on Linux. You need to use Linux to mount NFS. And yes the Windows NFS client performance are horrible...
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1