I have an issue with Outlook after the latest update that affect the reading pane when connection speed is slow. I have set the checkbox to Use Outlook Cached Exchange Mode, so technically there shouldn't be anything Outlook is "looking" for from the servers when loading messages, since they are supposed to completely downloaded already. So I cant see as to why there should be any lag when in Online Mode versus Offline mode, but there definitely is. When in Offline Mode I do not get anything extra in my reading pane so I dont understand why this should be impactful.
I had this issue a couple years ago with the very first installation of Outlook 2016. I removed and reinstalled the software and the issue went away. Then Outlook auto updated and the issue came back. I once again removed and reinstalled, disabled automatic updates, and for a couple years now the issue had not returned. One day I decide to go ahead and re-enable updates to try and get the product up to date, and now I have this issue again.