Please find my comment for your each points as under
Point 1:
Current latest versions of plugins installed
Pro : 2.3.8
Cornerstone : 3.3.8
Essential Grid : 2.3
Point 2:
We are on staging server and there are no CDN or cache applied to staging server
Point 3:
We deactivated plugings to check conflict but it did not make any difference
Point 4&5:
Switched to parent theme
Point 6:
We are on nginx server so .htaccess does not make any difference. Though we renamed htaccess. Also we did this “Then in WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.”
Point 7:
We are on staging server so there are no caching on this server.
Still there are no improvements. Can you please take a look at the site admin?
Please Help
Thanks