It's more complicated than that. Specifically having a mix of any CPU and I/O bound threads doesn't play out as nicely:

See this:

http://dabeaz.blogspot.com/2010/02/revisiting-thread-priorit...

Or even better for a demo watch David Beazley's Pycon 2015 video (It is an awesome video even if you don't care about the GIL or socket programming).

Looks like this is the video referred to: http://pyvideo.org/video/3432/python-concurrency-from-the-gr...

Yap, thank you for finding. I was just being lazy.