My work mail is Microsoft Office 365, which I access using Mail.app on the Mac. Recently, I noticed performance had become so slow as to become painful. Mail downloaded just fine, but when I clicked on a message it took forever to open, and when I marked a message as read it took forever for the message status to change.
The solution: Go into “Preferences” and make sure “Load remote content in messages” is unchecked. Loading remote content means the Mac has to go out to the Internet and download images, which takes time.
It’s probably a good idea to uncheck that for security reasons as well as for performance.
I don’t recall whether that box is checked or unchecked by default. Previously, I had it checked – messages set to download remote content automatically. And that’s what was slowing down my Mail performance. I unchecked it, and mail performs just fine for me now.
If I want to see remote content, such as images, for a particular message, I can click a button on the top of each individual message, and the remote content downloads quickly enough.
Most of the time I don’t bother. I don’t bother reading about 99% of the email I receive anymore.