For better comparison with my numbers, I ran your scripts on my machine:

  $ python3 yes3.py | pv -r > /dev/null
  [18.4MiB/s]
  $ python2 yes2.py | pv -r > /dev/null
  [10.2MiB/s]

In both cases, a 4KiB buffer is used by python. That's still way slower than the equivalent rust code with a 4KiB buffer (use BufWriter::with_capacity(4096, stdout.lock()) instead of BufWriter::new(stdout.lock())).

Out of curiosity:

yes2.py:

  import os
  stdout = os.fdopen(1, 'wb')
  while True:
      stdout.write('y\n')


  $ python yes2.py | pv -r > /dev/null
  [9.12MiB/s]

  $ pypy yes2.py | pv -r > /dev/null
  [45.5MiB/s]

So pypy does a good job of speeding it up.

Off a quick 9 second run, python2 with profiling:

     ncalls  tottime  percall  cumtime  percall filename:lineno(function)
          1    4.272    4.272    9.301    9.301 yes2.py:1(<module>)
   30856080    5.029    0.000    5.029    0.000 {method 'write' of 'file' objects}
          1    0.000    0.000    0.000    0.000 {posix.fdopen}
          1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}