> as far as I remember the GIL only affects Python objects. So if you use Numpy for operations, you can avoid the GIL.

Correct.

Threads cannot help Python-heavy code; they can help I/O-heavy or native-heavy code (assuming native code releases the GIL).

Even without multithreading, numpy programming can give amazing speedups. It's the reason why data science Python isn't dead in the water.