Hi!
We are running similar issues with one of our clients that was upgrading from CMS 13 to CMS 16, but the performance degradation only occurs on large block grid items in the Umbraco Cloud environments, not locally.
The RC4-fix was suppose to address this issue but we still experience publishing times in the backoffice between 10-15 seconds on some pages.
The errand is currently ongoing with the Umbraco black ops team and hopefully more information regarding this can be found.
We have been looking through the solution locally with no finds regarding anything suspicous, so far!
Hopefully this info finds someone that are in a similiar situation as we are.