Comment by mswphd

not at all. even the previous O(n \log n) multiplier is almost never used in practice. In general fourier-based multipliers only start being useful at a minimum when you are multiplying 1000-digit numbers, but even that threshold is low/contentious. they have a definite advantage for ~10,000-digit numbers. this likely has larger overhead, so is only plausibly useful for even larger numbers. you do not often have to multiply these extremely large numbers.