[dajonker]

There's no reasonable way to compare performance between PostgreSQL and MongoDB because they're not the same kind of product and they don't have a similar query interface.

You can take a particular application and compare performance when using two different databases on the back-end, but then the database itself might not necessarily be your bottleneck, it might also be the way the application is written. Because the database is only a part of the equation, the comparison also doesn't tell you anything about the performance of any other application.