XenServer VM poor network performance

In a recent project we were seeing very slow printing on the new Windows 2003 print server VMs created on XenServer. Some print jobs were taking 20 minutes to print where they should only have been taking 20 seconds. It was easy to see that the print job spooled quickly to the print server, but from the print server to the printer the job went at a snail’s pace.
I can’t take responsibility for finding this fix (thanks Reece!):

HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
DisableTaskOffload = 1 (DWORD)

After some testing I created a group policy which applied this fix to all XenServer VMs in the domain (using WMI filtering).

We were using XenServer 5.5 so I don’t know if the same issue applies on XenServer 5.6 with the latest Xen Tools.