Cached Exchange mode keeps a copy of your mailbox on your local computer instead of having to contact Office 365 every time you switch between emails. When a new email is delivered to Office 365 it will sync to the local copy of your mailbox on your computer. Because of this sync you will notice emails getting delivered to your iPhone or iPad first and then after a short delay getting delivered to your Outlook.
To enable Cached Exchange Mode
It may take a little time to download a local copy of your mailbox, but after that Outlook should have better performance.