This can cause serious issues, especially when using PST files.
But you never know what level of skills you're dealing with, so we'll cover all the bases.
In the end, you should have a much faster Outlook experience. There are times when Microsoft might update Exchange.
This option (obviously) is available only when connecting Outlook with an Exchange server.
When the Outlook PST file gets out of hand, a built-in tool can keep the size of that file under control.
Many people don't realize that with Windows updates, the updates for Microsoft Office are also "hidden." But it's not just the Office updates that can help speed up Outlook. In some cases, those updates can either break or cause problems with the communication between Outlook and Exchange.
Or Microsoft might issue a patch to the application to resolve a speed issue or security hole.
This can go a long way toward speeding up your Outlook experience because Outlook doesn't have to read its data file across a network.
Instead, all it has to do is read the locally stored data file.
When you connect Outlook with either IMAP or POP3, you should have Outlook set up to download the complete message (instead of just the header).
If you do this, Outlook won't have to sync with the server every time you click on a new item (as it will already be in the data file.) How you do this will depend upon which version of Outlook you're using.
While Online Mode maintains a direct connection to an Exchange server, Cached Exchange Mode creates a local copy of the mailbox data stored on the Exchange server in an offline data file (file).