Form submission sometimes takes up to 15 to 20 seconds when going to the "thank you" landing page.
Here is an example form link example https://www.masterclassmanagement.com/cert-submission.html you can use to test. Also, whenever it takes a long time to go to the thank you/landing page it coincides with not going to the email notification. I can only see the submission over the Form Manager. This has been an issue for 2 months now.