Writing separate lines:

$ ./python -m timeit -s 'import sys' -s 'with open("setup.py") as f: s = f.readlines()' 'sys.stderr.writelines(s)' 2>/dev/null
200 loops, best of 5: 1.07 msec per loop

$ ./python -u -m timeit -s 'import sys' -s 'with open("setup.py") as f: s = f.readlines()' 'sys.stderr.writelines(s)' 2>/dev/null
Unpatched:  50 loops, best of 5: 5.89 msec per loop
Patched:    100 loops, best of 5: 3.32 msec per loop

Writing separate characters:

$ ./python -m timeit -s 'import sys' -s 'with open("setup.py") as f: s = list(f.read())' 'sys.stderr.writelines(s)' 2>/dev/null
10 loops, best of 5: 30 msec per loop

$ ./python -u -m timeit -s 'import sys' -s 'with open("setup.py") as f: s = list(f.read())' 'sys.stderr.writelines(s)' 2>/dev/null
Unpatched:  5 loops, best of 5: 49.2 msec per loop
Patched:    2 loops, best of 5: 137 msec per loop