While it might be *web/email, this program has been out for years and this is the first report of anybody encountering slowness.
Perhaps its related to the email/smtp server you are connecting to?
Often web sites do what is called 'Tarpitting' which is they intentionally slow down email submission to help reduce spam. If a mail server ran at full speed a spammer could send hundred of thousands of emails a minute, whereas if each email waits 1-2 second before acceptance then the most you send is 30-60 per minute with minimal impact to the end-users,
Another thing that may slow up email is if the email server is checking the in/outbound addresses. I know our mail server does -- that is both sender and receiver email addresses are pre-validated to see if the server can determine if they exist. It also does a spam check both on the inbound server (to see if its blacklisted) and runs the email contents through a spam/anti-virus filter. These checks all take time -- more for larger emails.