Currently BytesIO.writelines() uses BytesIO.write() which returns number of written bytes as Python ints, but these ints are not used by BytesIO.writelines(), so avoiding creation of these ints can improve performance of BytesIO.writelines() especially for a large number of small lines.

Benchmarks:

python -m perf timeit --compare-to ~/tmp/cpython-master-venv/bin/python -s "from io import BytesIO; lines = [b'']*10000;" "BytesIO().writelines(lines)"                                          
/home/sergey/tmp/cpython-master-venv/bin/python: ..................... 153 us +- 3 us
/home/sergey/tmp/cpython-venv/bin/python: ..................... 126 us +- 4 us

Mean +- std dev: [/home/sergey/tmp/cpython-master-venv/bin/python] 153 us +- 3 us -> [/home/sergey/tmp/cpython-venv/bin/python] 126 us +- 4 us: 1.22x faster (-18%)

python -m perf timeit --compare-to ~/tmp/cpython-master-venv/bin/python -s "from io import BytesIO; lines = [b'b']*10000;" "BytesIO().writelines(lines)"
/home/sergey/tmp/cpython-master-venv/bin/python: ..................... 164 us +- 2 us
/home/sergey/tmp/cpython-venv/bin/python: ..................... 142 us +- 1 us

Mean +- std dev: [/home/sergey/tmp/cpython-master-venv/bin/python] 164 us +- 2 us -> [/home/sergey/tmp/cpython-venv/bin/python] 142 us +- 1 us: 1.16x faster (-13%)

python -m perf timeit --compare-to ~/tmp/cpython-master-venv/bin/python -s "from io import BytesIO; lines = [b'b'*10]*10000;" "BytesIO().writelines(lines)"
/home/sergey/tmp/cpython-master-venv/bin/python: ..................... 387 us +- 6 us
/home/sergey/tmp/cpython-venv/bin/python: ..................... 365 us +- 8 us

Mean +- std dev: [/home/sergey/tmp/cpython-master-venv/bin/python] 387 us +- 6 us -> [/home/sergey/tmp/cpython-venv/bin/python] 365 us +- 8 us: 1.06x faster (-5%)

python -m perf timeit --compare-to ~/tmp/cpython-master-venv/bin/python -s "from io import BytesIO; lines = [b'b'*100]*10000;" "BytesIO().writelines(lines)"                                                    
/home/sergey/tmp/cpython-master-venv/bin/python: ..................... 319 us +- 5 us
/home/sergey/tmp/cpython-venv/bin/python: ..................... 305 us +- 16 us

Mean +- std dev: [/home/sergey/tmp/cpython-master-venv/bin/python] 319 us +- 5 us -> [/home/sergey/tmp/cpython-venv/bin/python] 305 us +- 16 us: 1.04x faster (-4%)

python -m perf timeit --compare-to ~/tmp/cpython-master-venv/bin/python -s "from io import BytesIO; lines = [b'b'*1000]*10000;" "BytesIO().writelines(lines)"
/home/sergey/tmp/cpython-master-venv/bin/python: ..................... 1.13 ms +- 0.02 ms
/home/sergey/tmp/cpython-venv/bin/python: ..................... 988 us +- 88 us

Mean +- std dev: [/home/sergey/tmp/cpython-master-venv/bin/python] 1.13 ms +- 0.02 ms -> [/home/sergey/tmp/cpython-venv/bin/python] 988 us +- 88 us: 1.14x faster (-12%)

python -m perf timeit --compare-to ~/tmp/cpython-master-venv/bin/python -s "from io import BytesIO; lines = [b'b'*10000]*10000;" "BytesIO().writelines(lines)"
/home/sergey/tmp/cpython-master-venv/bin/python: ..................... 38.7 ms +- 0.1 ms
/home/sergey/tmp/cpython-venv/bin/python: ..................... 38.2 ms +- 0.1 ms

Mean +- std dev: [/home/sergey/tmp/cpython-master-venv/bin/python] 38.7 ms +- 0.1 ms -> [/home/sergey/tmp/cpython-venv/bin/python] 38.2 ms +- 0.1 ms: 1.01x faster (-1%)