Slow Apple Mail in Big Sur when online

I’ve been having terrible issues with Apple Mail in Mac OS Big Sur.
I have two accounts (Gmail and iCloud). When browsing between folders or selecting mail messages (new or already read) Mail often beachballs for up to a couple of minutes.
The behaviour only happens when online (either wifi or ethernet). If I am offline then using mail is fast. My internet speed is fairly good.
I have tried disabling one account at a time, and re-built mailboxes but have not seen any improvement.
Is there anything else I could try?

In my experience, this often happens when Mail is downloading contents of folders that you haven’t looked at in a while, especially after a new install or on a new machine. Any chance this is just your Mac trying to catch up to the current state of the (email) world?

(And if not, I can heartily recommend MailMate as an alternative)

Thanks for the tip. I’ve looked at folder downloads but I dont think that is it. I’m considering doing a clean install to see if that helps.
Having tried it, I would love to use MailMate! But I’m rather reliant on Daylite’s mail plugin for my business so I’m stuck with Apple Mail for now.