Both fast and very slow scipy.signal.resample with the same input size

Question

According to the documentation of scipy.signal.resample, the speed should vary according to the length of input:

As noted, resample uses

Answer 1

The resampling process consists of FFT (input size), zero-padding, and inverse FFT (output size). So an inconvenient output size will slow it down just as much as an inconvenient input size will.

just to add that this is the case for upsampling only. for downsampling the process is: FFT -> multiply -> iFFT -> downsample. so in downsampling, the FFT/iFFT has nothing to do with the output size, only the input size.