Windows 11 ships with two versions of Outlook. There is Outlook Classic, the long-running Win32 desktop app built for power users, and there is the new Outlook, which Microsoft is pushing as the future of email on Windows. The newer one is built on WebView2 and is, in essence, a browser window that loads Outlook.com.

Old Outlook downloads all your stuff when you sync, so it's immediately available when you try to look at it. New Outlook just being a wrapper around a website probably means it's asking the server for things as you select them.