Hi,
we encounter the behaviour on various sites. The cornerstone editor is very slow, sometimes it takes 30 seconds and more to save a page. If I add a classic feature list item, it takes long to display. Please note that the sites run on a server with great performance, lots of ram. The PHP memory limit is 512MB per domain.
I tried various fixes which were here in the forum without success.
The second issue:
If I add a “classic feature box” with text and save it in cornerstone, everything is fine the frontend.
If for some reason I need to update the page (not in cornerstone but in normal view) by hitting save, the texts in the feature box are gone, until I resave the page in cornerstone.
The issue is, that if we have to change a page settings under the normal editor (e.g. autolinker, Seo settings etc.) we have to hit save in the normal view. However it breaks the classic feature box.
We tested it in Chrome and Firefox.
I created an Admin Account for you to test.
Thank you,
Matt

In further tested it - it happens only in chrome, not in firefox.