All four cores at 25% is the same than one single core at 100% (I assume the extra 5% is other activity on your machine). Unfortunately, due to the GIL you cannot efficiently use all four cores simultaneously with Python (or at least with CPython which is the usual implementation...). So you have to either find a non GIL-limited Python, or spread your work load across processes instead of threads, which isn't too hard to do with the multiprocessing module.
Unless noted otherwise, code in my posts should be understood as "coding suggestions", and its use may require more neurones than the two necessary for Ctrl-C/Ctrl-V.
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