Not only that, because of the way that nested objects are indexed, joining the nested documents to the root document at query time is fast—almost as fast as if they were a single document.
In particular, joins should be avoided. nested can make queries several times slower and parent-child relations can make queries hundreds of times slower. So if the same questions can be answered without joins by denormalizing documents, significant speedups can be expected.
I found this 2 references of nested objects on Elastic documentation, but don't know what which one is true.