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.
You can get amazing performance results if you don't care about data persistence. Anyway when postgresql introduced JSONB there was a benchmark for CRUD operations, can't find it now, because it was long time ago, but I remeber postgresql was doing circles around mongodb.
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.