In my experience object persistence rarely needs SQL or other "relational additional stuff" underneath. Typically I need to store objects and their mutual relations persisted as if they were just kept in memory. Have often thought about how weird it was that you could not just persist objects "as they are" without additional mappings etc.
I saw a demo of Realm and had a talk with one the developers a couple of weeks ago. As the demo and our talk progressed I found myself nodding more and more excited. As I understand Realm it seems to be based on a very natural and straightforward principle. It can't NOT be efficient...
I can't wait to try this out in a real project!