What changes can I make that would help system performance during peak period

I would correlate the time of day of this message with the time of day in the log. This message probably appears in the log too so it will be easy to spot.

Since this is smtpd(8postfix), my guess would be that it's trying to handle a very large message or an abnormally-long connection from a malevolent actor (mail is a very common attack vector). If it's either of these, you'll want to take mitigation steps. For the former, in /etc/postfix/main.cf, you can set

# message size limit (25 Mib)
#
message_size_limit = 26214400

to something smaller than what it currently is -- this affects both incoming and outgoing mail. For the latter, what you do will depend on the threat/actor -- check out fail2ban(1)…http://www.fail2ban.org . If your server is an open relay, you'll probably want to shut that off. See:

• https://blog.mailtrap.io/test-smtp-relay/
• https://anandarajpandey.com/2019/12/30/postfix-how-to-prevent-open-relay/

If it's neither of those, and your system is just really busy, you can try following some of the advice listed here:

http://www.postfix.org/STRESS_README.html

Last, but not least, you can try changing to a bigger Linode to acquire more resources. If you don't change the size of your disc, you can migrate back to a smaller Linode easily with no data loss if your experiment doesn't pan out.

-- sw