Indeed, and I am not claiming otherwise. However, it is well understood (see bugs.python.org and search for "GIL") that the GIL causes performance problems in o

Indeed, and I am not claiming otherwise. However, it is well understood (see bugs.python.org and search for "GIL") that the GIL causes performance problems in otherwise correct and performant Python code. For example, such programs can run more slowly if there are more cores available than if there is only a single core available. Various workarounds to the GIL implementation in CPython have been proposed to address these problems, and it's better in Python 3.X, but the problems still exist. And one still cannot write true multithreaded programs in Python using the language's standard thread library. To use all the cores, you have to resort to the processing module, which relies on processes rather than threads; this vastly increases the memory consumption, and makes the resulting programs less idiomatic as well. (You have to pass only pickle-able stuff to your child processes, for example.)