Oops... It hadn't jumped at me earlier, but the patch is actually problematic performance-wise. The reason is that it doesn't buffer data at all, so small readintos become slower (they have to go through raw I/O every time):

$ ./python -m timeit -s "f=open('LICENSE', 'rb'); b = bytearray(4)" \
  "f.seek(0)" "while f.readinto(b): pass"
-> without patch: 2.53 msec per loop
-> with patch: 3.37 msec per loop

$ ./python -m timeit -s "f=open('LICENSE', 'rb'); b = bytearray(128)" \
  "f.seek(0)" "while f.readinto(b): pass"
-> without patch: 90.3 usec per loop
-> with patch: 103 usec per loop

The patch does make large reads faster, as expected:

$ ./python -m timeit -s "f=open('LICENSE', 'rb'); b = bytearray(4096)" \
  "f.seek(0)" "while f.readinto(b): pass"
-> without patch: 13.2 usec per loop
-> with patch: 6.71 usec per loop

(that's a good reminder for the future: when optimizing something, always try to measure the "improvement" :-))

One solution would be to refactor _bufferedreader_read_generic() to take an existing buffer, and use that.