Comment by pron

It may well be the case that it's not an apples-to-apples comparison, but as someone with over two decades of experience in both Java and C++, I find it not only unsurprising, but as a case of both Java and Rust doing exactly what they're designed to do.

Rust is designed to be a low-level language, i.e. a language with maximal control with all of its pros and cons (albeit with memory safety, which C++ doesn't have), while Java is designed to address the performance issues low level languages have, particularly as they get larger, due to their control constraints. Without such constraints, it is easier to offer better performance for less effort especially as programs grow.

In that particular program I was told that the differences were due to needing more locks in the Rust version. As has always been the case, they managed to achieve parity with much more effort (that is expected to continue over the lifetime of the software), but again, this is the explicit tradeoff of the approaches.

Thirty years ago, and even twenty years ago (when Rust was first being designed) many still believed that more control is the only path to good performance, even if it comes with a lot of effort. Today it's clear that it's not the only path, and the debate is mostly around which program and workload patterns that happen to work better with one approach or the other are more common.