It is I/O bound, but Python 2 and 3 do I/O differently. From the same comment that you're referring to: > Python 2 emits writes of 4096 bytes, and python 3...

It is I/O bound, but Python 2 and 3 do I/O differently. From the same comment that you're referring to: > Python 2 emits writes of 4096 bytes, and python 3... 8193 bytes (edit: not a typo, this is 8KiB + 1).