It depends on what your measuring. I work on a project that gained significant speed improvements moving to MongoDB simply due to packing and unpacking the data

It depends on what your measuring. I work on a project that gained significant speed improvements moving to MongoDB simply due to packing and unpacking the data. MongoDB BSON serialization was way faster than the ORM when building up the data objects after fetching from the database. In our workload that operation actually dominated the workload not IO. This article is interesting because Postgres document store options may make it competitive to us when compared to MongoDB.