Outlook 2016 slow to connect under Parallels but not on the Mac side--Why?

iMac 2014 32GB member, El Capitan, Win 10 Home, Parallels 11

I use Office365 and have downloaded Outlook 2016 for Windows to my Parallels VM and Outlook 2016 for Mac for the Mac side. I connect to three exchange servers on three different accounts. It's the same servers and same accounts, whether I connect from Mac or from Windows.

Outlook for Mac connects promptly and starts downloading new messages in a few seconds. Outlook for Windows takes forever to connect and retrieve messages. On the bottom status bar, I get "Trying to Connect" messages.
I am of course using the same Internet connection which does not involve a VPN. I have stripped out all Outlook Windows add-ins except for Exchange, OneNote and Dragon.

I can't understand why there's such a difference connecting to the same exchange servers. How do I troubleshoot this?