I have tried several approaches to speed up Ripple Noise Removal and more the CPU cores. Unfortunately, the most promising solution,
using pathos.multiprocessing.ProcessingPool, ran into an error: [WinError 87] The parameter is incorrect. Something similar here points at dill on Windows.
For now, I will just keep using single-core RNR, which is not terribly slow (probably ~10 minutes per video).