Apple Mail delays in getting mail content once an email is opened

I use Apple Mail on an iMac, with a primary email address of _____@twc.com but also get some mail like Apple News from @icloud.com There is a delay in getting mail content. The sender and subject show in preview immediately but not the content. Even after opening, the content is blank for a minute or so. So after downloading the mail, I go browse for awhile to wait for the content to appear.

This does not always happen. It happens when a lot (10-20) of messages come into the Inbox, like in the morning, but not when I send myself a test message.

This started not too long ago, perhaps after the update to Monterey.