For more detailed information on backup failures and performance issues see Unitrends KB 5062 - Backup Failures and Performance Issues
The recommendation from XenServer Support is to disable ALL Offloading (File Level Slow Backup Performance on Xen Guest Windows Operating System recommends disabling the Large Send Offload however, that by itself may not make enough of a difference).
If you have Powershell 3.x or above you can run the powershell command:
Disable-NetAdapterChecksumOffload -Name *
This will hit all adapters and all relevant settings. You can confirm your success with the corresponding
Offloading causes performance drops when the Xen virtual machine is not being actively used for non-backup tasks.