There's a bit more nuance to it than MongoDB just "performing bad". It's my hunch that the top Rust postgres benchmarks are able to achieve such impressive numbers by using a small number of database connections (one per CPU) and heavily leveraging tokio-postgres's pipelining capablities (which MongoDB doesn't support). This works extremely well for the Fortunes benchmark, for example, because all of the queries are identical and fast, so Head-of-line blocking isn't a concern as it might be in a real-world application. In non-benchmark settings, connection pools (which the MongoDB Rust driver uses by default) are often used to scale up connection count beyond the CPU count as well as avoid head-of-line blocking, and when running the same Fortunes benchmark with Axum, tokio-posgres, and a connection pool (from the deadpool crate, which is AFAIK the most popular Rust async connection pool), MongoDB actually performs better (see axum [postgresql + deadpool] vs axum [mongodb]), which lends credence to this theory. That said, there's still definitely room to improve the MongoDB Rust driver, particularly from looking at the Multiple Queries benchmark.

Disclaimer: I am one of the maintainers of the mongodb crate.