Fun fact:

  $ python yes.py | pv -r > /dev/null
  [11.2MiB/s]

  $ python3 yes.py | pv -r > /dev/null
  [4.95MiB/s]

As for why the naive rust version is slower, it's because without adding a BufWriter in rust, stdout is line-buffered, so each line emits a write system call, while with python, stdout is buffered. Python 2 emits writes of 4096 bytes, and python 3... 8193 bytes (edit: not a typo, this is 8KiB + 1). That's the likely cause for it being slower.

Edit: A minimal version of the naive rust version would be:

  fn main() {
    loop {
      println!("y");
    }
  }

On the same machine as with the python tests above, I get:

  $ ./yes | pv -r > /dev/null
  [4.81MiB/s]

which is actually as slow as python 3, despite doing 4 thousand times more system calls.

A version with buffering would look like:

  use std::io::{stdout,BufWriter,Write};

  fn main() {
    let stdout = stdout();
    let mut out = BufWriter::new(stdout.lock());
    loop {
      writeln!(out, "y").unwrap();
    }
  }

And produces 129MiB/s on the same machine. And that's strictly doing the same as what the python version does (with a default buffer size of 8KiB, apparently).

And fwiw, on the same machine, both GNU yes and the full rust solution from OP do 10.5GiB/s.