My use case was a program involving many calls to an external program that generated an XYZ file format to read in (computational chemistry). It's likely I was doing something wrong or inefficient, but I remember the whole process was rate-limited by this step in a way that Python wasn't.
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.
