PostgreSQL as Schemaless Database [pdf]
I want to admit that conclusion in presentation can be interpreted vise versa: MongoDB almost is fast as PostgreSQL. Also, notice that comparsion was done with Mongo 2.2, in 2.4 JS engine was changed to V8, so there is a lot of speed improvements. And compare PG have 18 years of development vs 5 of MongoDB. So what I want to say? Difference is not so big after all.