For future googlers looking a solution to the indefinite checkout loading issue.
What helped me was disabling “Cimy Swift SMTP” Plugin. Tried disabling all plugins and it worked, so after that it was just a matter of narrowing it down.
Yeah, as a developer myself I know how frustrating it is to try to fix an issue that you can’t replicate. If you want me to test drive the new version you can send it to me at yuriysATgmail.com