Hello same situation here.
My site is hosted at hosteurope.
What I've tried so far:
1. Ensure that PHP version ist 7 or later (7.1 in my case)
2. Update all X (6.0.4) and Cornerstone (3.
3. Caches cleard, including browser cache
4. I don't use CDN
5. Deactivated all plugins (except Cornerstone)
6. Check that custom CSS, JS and templates are removed
7. Inserted the lines:
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
in my wp-config.php
Unfortunately cornerstone still loads very slowly. Once it has started, it sometimes does not show the content of the pages and freezes occasionally.
Thanks for helping,