https://www.bleepingcomputer.com/news/microsoft/microsoft-shares-workaround-for-outlook-freezes-slow-starts/

Microsoft is working to address a known issue affecting Outlook for Microsoft 365 customers, causing slow starts and freezes as if Offline Outlook Data Files (OST) are being synced right after launch.

However, no new OST files are being created, and no new entries associated with this behavior are added to the Application event log, according to Redmond's investigation.