Theme doesn't work with latest version of Wordpress

I purchased Motors theme more than six months ago. Everything worked without problems.
WordPress releases a new version of the platform and I upgrade. Suddenly, my Customizer stops working. When I open it, i have to wait good 30-40 seconds until the menu is responsive.

I brought this up to the author and they are saying that there is no issue on their end. The refuse to help me because my support has expired and asking me to renew. I would like to give them a login to check it out, but they won’t accept it unless it comes through a ticket, which I cannot submit because my support expired.

What are my options?

The theme may not be compatible with the latest version of the WordPress. You need to downgrade it to the previous version

Try to install it on a clean WP installation without any 3rd party plugins to make sure there is not some sort of conflict.