I’ve implemented in Python a recursive FFT function and an iterative one. They’re both correct (they both pass all unit tests) but, when I plot the complexity curves, I find that the iterative version is slower than the recursive one (the curves are both O(n lg(n)) but the iterative one is shifted up a little). Can you help me identify what, in the following code, is slowing down my iterative FFT method ? Am I using ThreadPoolExecutor wrong ?

I faced a problem with the fft in scipy and I’d like to explain this using an example.