radiumsoup

Found it:

On my mail host (same as the web host for this setup), /etc/hosts file did not reference the full domain ‘example.com’, it only had the host name ‘example’ – so sendmail timed out waiting to get DNS resolution for an unresolvable name. (It was configured to send mail from [email protected].)

First line of old /etc/hosts:
127.0.0.1 localhost example

First line of new /etc/hosts:
127.0.0.1 localhost example example.com

Instant redirect to “thank you” page after that.

The “Enable checking” box was not checked. Yes, it happens when I do a test email from the form edit page as well. Took 120 seconds exactly (first time I timed it with a clock instead of just counting in my head… I guess I must count seconds slowly.) Confirmed exactly 120 seconds on both the test email form and the live public facing form.