I have a Fastmail IMAP account using the legacy servers too. No connection problems or delays when sending. I am configured to have the SMTP server save a copy, rather than upload a copy via IMAP (using Thunderbird), but that doesn't effect the connection to the SMTP server.
Their main servers are located at New York Internet (NYI) in Bridgewater, New Jersey, USA. Since I'm in the USA and you're in Canada we both should be using the same servers.
Do you have a similar problem reading messages using your IMAP account?
Perhaps your anti-virus software or some other security software is the culprit since it doesn't effect telnet. If you have your anti-virus scanner configured to scan outgoing messages I suggest you disable that feature. Its not worth the hassles it causes.
I don't have any antivirus other than the standard Windows 10 defender, and it is not scanning outgoing email
The consistent 30 seconds delay sounds like some sort of a timeout on FM's side.
Try configuring Thunderbird not to save a message and send a message. See if you have the same delay. I'd like to rule out any possibility, not matter how remote, that problems uploading a copy of your message is causing your problem. The reason why I configured Fastmail to save a copy of the message instead of Thunderbird is because frequently sending (via SMTP) worked okay but the uploading (via IMAP) failed. They're using different protocols, but the same server, and operate independently. I've actually had it save a copy of a sent message that it failed to send
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