Just FYI, MongoDB storage also uses mmap internally.

So we are comparing here "just mmap" with "mmap + all that connection handling, query parsing, JSON formatting, buffering, indexing, whatever stuff that MongoDb does".

And no, MongoDB is effectively never a cheap solution. They are used because they are super convenient to work with, with all things being JSON documents. But all that conversion to and from JSON comes at a price. It'll eat up 1000s of CPU cycles just to read a single document. With raw mmap, you could read 1000s of documents instead.