Trying to debug the issue, I deleted my problematic Filed Group, the only with a terrific delay which sometimes is even able to crash Chrome.
After deleting, I started recreating the fields group.
1. First I set a Post Object field.
2. Second a Flexible field.
3. Inside the Flexible, in the first Layout, I placed a Relationship field.
That was already enough to start experiencing a serious delay with page freeze.
All of my other 29 fields groups still show no issues at all so I guess there should be something closely related to this kind of grouping.
