Outlook 2013 introduced the concept of hybrid mode (sometimes called Exchange Fast Access) to allow clients configured in cached Exchange mode to fetch data direct from the server when possible. The mode works well, except when you're connected on a poor network as the attempts to fetch data from the server might cause Outlook to hang. A registry setting gives a way to force Outlook to operate in classic cached mode and use the OST exclusively when it needs data.
https://office365itpros.com/2019/08/14/outlook-hybrid-mode-flaky-network-conditions/
