Mail messages don't load on first try often, sometimes never

Hi,

I've had this issue with Mac Mail for months and I cannot find a solution after searching the community.

This is what is happening -- when a new mail arrives, it appears in my Inbox but when I click on the email to read it, it does not load the body of the email immediately. It would take about 3-5 minutes before the body of the email appears and only after several retries. Even opening of old emails has this behavior. This is a Mac Mini with High Sierra v10.13.5.

Any tips would be greatly appreciated.

Thanks!