Excessive delay after typing sudo?
Looks as though the optional pam_ck_connector.so is hanging not only sudo but other auth processes (eg roundcube/dovecot/imap).
From here by erikdhansen (http://ubuntuforums.org/member.php?u=36598)
(Copied in case ubuntu reorganise their forums!)
"Re: SSH Login takes long time (no DNS Problem)
So I was having this same problem and it wasn't the DNS issue that everyone points to. It ended up being a problem with PAM.