Hi,

Thank you for loving our theme.

Strange, I've tested the customize changes on a live site and I didn't see it slow like that. So, maybe you have a plugin caused this issue, have you tried deactivate all plugins and check it again? If it works, let's enable each plugin to check what's plugin caused this issue.

And here is a tip for you: Let's change everything ( can be change on many options ) as you want and save it - and check the changes on the front-end. It's faster than wait the preview from Customize.

Best Regards,

PenciDesign