Alternative SMTP configuration performance

Over at the Mauteam website I can see a chart for capacity planning that is a great place for me to start.

One question I have is related to the SMTP option. I am assuming that their configuration for SMTP is using the localhost MTA which will directly connect to the recipient’s server. This would mean that the local server is generating a DKIM signature and initiating an encrypted session, both of which would certainly increase CPU usage vs. the API option.

However, I am looking at using the localhost MTA only to send mail to a private network smarthost, so there would be no DKIM key signing by the Mautic server nor any initiation of an encrypted session.

With this configuration the server is technically using “SMTP”, but wouldn’t the resources look something similar to the API?