It's surprising that something as simple as reading a file is slower in the Rust standard library as the Python standard library. Even knowing that a Python standard library call like this is written in C, you'd still expect the Rust standard library call to be of a similar speed; so you'd expect either that you're using it wrong, or that the Rust standard library has some weird behavior.
In this case, it turns out that neither were the case; there's just a weird hardware performance cliff based on the exact alignment of an allocation on particular hardware.
So, yeah, I'd expect a filesystem read to be pretty well optimized in Python, but I'd expect the same in Rust, so it's surprising that the latter was so much slower, and especially surprising that it turned out to be hardware and allocator dependent.