For some reason, it appears to be better the past day.it's a Gmail IMAP issue (first identified in March), which is impacting certain Gmail inboxes >3000 messages.
One of mine - primary - has it.
My other does not.
My wife's does not.
Apple is looking into it - escalated to engineering (because Apple people are having the issue, NOT because of me specifically - I'm a nothing).
Check > updated just now is back down to 1-2 seconds.
Mail-battery-use-while-idle still spikes in the middle of the night.