As per my knowledge, nested type mapping has more impact in terms of indexing over flat type mappings. Since Lucene does not have any concept of nested object types, and everything is stored as flat objects. So there is an additional operation performed at the indexing time. ES works best for flat documents. For more details, you can refer to the following links.
- https://www.elastic.co/guide/en/elasticsearch/guide/current/nested-objects.html
- https://www.elastic.co/guide/en/elasticsearch/reference/current/nested.html#_how_arrays_of_objects_are_flattened
I don't think so there are any significant query performance impacts for nested documents. It is usually the indexing operation that suffers the most. A similar older post can be found here.