Usually, slow form submission/redirection is due to the actions that are being processed on submit such as emails being sent, data being sent via API, Connections to Mailchimp etc etc.
Could you please try toggling all your form actions off and testing if the form submits quickly then? If it does, toggle your form actions back on one at a time, testing between each activation so that you can figure out which form action is causing the delay. Once we pinpoint the specific action causing problems we can troubleshoot it.
Let me know what you discover.