Sorry for the inconvenience, we found that many people have reported the same issue.
https://social.technet.microsoft.com/Forums/en-US/7b412a7a-3377-46f7-95f8-cd42d24451a0/windows-server-2016-updates-slow?forum=ws2016
So far, there is no particularly good way to improve.
I will submit this case to Microsoft via our channel.
You can use the Feedback Hub app to share your problem.
Sorry for the inconvenience.
Hope above information can help you.
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