Well, the synchronizing speed may be due to slow internet speed but there is also an option in the mail settings where you can edit some few things and increase the speed with slow synchronization issues with Outlook IMAP folder. Here is how you can do it.
- Open your Outlook mail, Go to to Send/Receive, then open Send/receive groups.
- Look on the right side of the send/receive groups window and search for the Edit option and select the Edit option.
- Scroll down and look for “Get folder unread count for subscribed folders” and uncheck this option.
- Now go to received mails, check the box option “Use the custom behavior defined below”
- Go to inbox and leave the other folders unchecked.
- Click save and close the window.
- Try a send/receive to check if your Outlook is faster than before