Comment # 5 on bug 1001854 from Dr. Werner Fink

Even if I've never used this as my mail folders are huge and shared over NFS

14.3. Calculating Mailbox Message Counts

   If $mail_check_stats is set, Mutt will periodically calculate the
   unread, flagged, and total message counts for each mailbox watched by
   the mailboxescommand. This calculation takes place at the same time as
   new mail polling, but is controlled by a separate timer:
   $mail_check_stats_interval.

   The sidebar can display these message counts. See $sidebar_format.

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3.144. mail_check_stats

   Type: boolean
   Default: no

   When set, mutt will periodically calculate message statistics of a
   mailbox while polling for new mail. It will check for unread, flagged,
   and total message counts. Because this operation is more performance
   intensive, it defaults to unset, and has a separate option,
   $mail_check_stats_interval, to control how often to update these
   counts.

3.145. mail_check_stats_interval

   Type: number
   Default: 60

   When $mail_check_stats is set, this variable configures how often (in
   seconds) mutt will update message counts.