You will often hear that Python is not good at parallelism and that you can only run on one CPU core at a time. This is likely referring to the aforementioned issues with threads and the GIL. Because you are limited to one thread, this means that thread-based tasks can only use one CPU core at a time (a single thread cannot run across multiple CPUs). Outside of Python, threads are a popular choice for parallelizing CPU-bound tasks because you are able to run a separate thread per CPU core simultaneously. However, with Python you must look for other ways to accomplish parallelism for cpu-bound tasks.