I've experienced the very same issue on multiple client sites - 'apolloisolate'. I believe the issue is Perfstack related, seems to be evident on every post-Perfstack instance I have checked and thus it even slows down the Orion Demo site by many seconds per page load. I currently have a support ticket open also. Support have not yet acknowledged to me that this is the issue but based on my findings everything is pointing in the 'apolloisolate' script's direction and I've provided them the associated DevTools Performance jsons to prove it.
Here's the DevTools Performance json I created, detailing the impact of the issue on the Orion Demo: DevTools Timeline Viewer
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Upgraded on 2/23 and performance has been awful since
I just installed a second web server last night on fresh hardware and still have the exact same issue