IO is thread-safe by default, but that does slow it down. There's a keyword argument to turn that off (if you know you're running it single threaded) and right

IO is thread-safe by default, but that does slow it down. There's a keyword argument to turn that off (if you know you're running it single threaded) and right now it's a rather large overhead. It needs some GC work IIRC to reduce that overhead.