Here’s a simple way to speed up Outlook when connected to accounts via IMAP.
In short, make Outlook work offline at all times except when connecting to send/receive messages.
Go to Send / Receive and click Work Offline
On the Outlook status bar it’ll show ‘Working Offline’.
Then go to Send/Receive | Send/Receive Groups. For the ‘All Accounts’ group (and any others) enable the ‘When Outlook is Offline’ setting ‘Schedule an automatic send/receive every nn minutes.
The default time is 30 minutes, you’ll want to reduce that to something between 5 and 15 minutes. This is how long Outlook will wait between connecting to send/receive new mail.
Why this works
Outlook’s IMAP has an annoying habit of checking the online store whenever you click on a message. Even though the email has already been downloaded and saved on the computer, Outlook will still check online for any changes. When online, Outlook pauses every time you move between messages which is damn annoying.
This leads many people to believe that IMAP is too slow, which is what Microsoft would like you to believe. Microsoft would much prefer people use their ActiveSync / Exchange Server system and IMAP is a rival to that.
The strange thing is that Outlook with IMAP is a lot faster when offline. In offline mode, Outlook doesn’t check the online store and just displays the selected message immediately.
The solution is to make Outlook work offline except for a brief time when it connects to send and receive new messages. You can click the Send/Receive All Folders button to check online without waiting for the next scheduled update.
If your whole computer is offline, then Outlook’s scheduled send/receive will skip without error until it finds a connection.