Customizer Lag

I’ve run out of ideas, so I’ll try here. The lag isn’t on everything, just loading Customizer on the home page. I tried Health Check & Troubleshooter, but even with everything turned off, it didn’t improve much. And I did need to turn back on the basics ( that gave the theme it’s pro features). I also tried it on a clone site, really turning it off. And I tried switching to a default theme (the older one that still used Customizer.)
The improvements were only slight. Nothing to pinpoint an exact cause.

Does anyone have any suggestions? I really thought it would be a plugin and was surprised that didn’t help. We use this theme on lots of sites mind you and the theme has never caused issues.