I have no idea, what's happens past 8 seconds.
Can you check your SMTP / LMTP / SPAMfilter-of-your-choice logs? Maybe you need to enable debugging logs.
I noticed slow sending of emails containing attachments and fixed it by configuring my SMTP server (exim4) to not scan outgoing messages for SPAM. For me, the issue surfaced when I upgraded my Debian Jessie server to Debian Stretch.
Kr,
Vincent