Hi @xhope
What about switching your outlook to cached mode then check if any difference? You could also try recreating the outlook profile for users migrated to Exchange 2016
I also see such issue was caused by incorrect DNS or virtual directory configuration, in a coexistence environment, make sure your have correctly configured them like below:
And DNS:
Please also take a reference at the link here:
https://support.intermedia.com/app/articles/detail/a_id/25189/~/outlook%3A-improving-performance
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
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