Changed active new-mail checks to every 2 seconds. Without interaction or a displayed mailbox change, checks slow to every 5 seconds after 30 seconds, every 15 seconds after 2 minutes, every minute after 5 minutes, and every 5 minutes after 15 minutes. Returning, interacting, or displaying a mailbox change restores frequent checks. Hidden tabs stop checking.
Faster active Mail checks
Mail checks every 2 seconds during active use and slows sooner when there is no activity.