Context: Outlook 365 "checks-in" frequently to ensure that what the user sees represents what is on AHS servers. This process can make Outlook seem slow if internet connections are not brisk.
Solution: Outlook 365 users on Windows devices can configure the software to use "cached mode", with significant performance improvements. This adjustment is not available, and not needed, with the Macintosh version of Office 365. And this mode is not relevant when Outlook Webmail is used.