You are reading this article because your Outlook email software is taking a long time to sync your IMAP folders.
This issue can often be caused by a slow internet connection because IMAP is a bandwidth hungry protocol so requires a steady and reliable connection.
The issue is compunded when you have a lot of subfolders and email in those sub folders.
One solution would be to increase the timeout settings in your Account properties and if that does not resolve it then you will need to disable the auto syncing of the subfolders and only sync your inbox on send and receive.
The screenshots below are taken from Outlook 2010 on Windows Edge but older versions will be similar.