This is already a staging site, the live site is on a completely different host, and yet the issue is exactly the same on both.
However, I have now created a second staging site, the URL and credentials are in the secure note. You will see that I have disabled all plugins and reverted to parent theme. I am not using a CDN.
Check out my test page to see that the issue is still there, despite all these troubleshooting steps. I even deactivated, deleted and re-installed MEC.
Please bear in mind that this is happening on two completely distinct hosting environments, Dreamhost and Siteground.
I duplicated your test on the new staging domain using a page called Cornerstone Test. Issue persists in this new page with parent theme, all plugins disabled, and memory maxed out.