Why is Apple mail so slow in opening/loading my email messages?

Then explain why this is happening to more than a few people?

My mail on iMac is slow, on my iPhone and MacBook, no issues and they are the same accounts.

As I sit here, I can read messages on my iPone while I wait more than a minute for the content to appear on my iMac. This issue started with a recent OS update.

I have turned off "privacy" options in preferences, no change.

All mail does is allow local control of a web mail service. As such more than likely it's a mail server issue with your provider or an internet issue with your ISP.