One of the surest performance killers for Sendmail users is when Sendmail is
configured to synchronously verify the recipient's host via DNS. If it does
this for messages posted to it from Mailman, you will get horrible
performance. Since Mailman usually connects via localhost
(i.e. 127.0.0.1) to the SMTP port of Sendmail, you should be sure to configure
Sendmail to not do DNS verification synchronously for localhost
connections.