The join() method of multiprocessing.Pool calls self._worker_handler.join(): it's a thread running _handle_workers(). The core of this thread function is:

        while thread._state == RUN or (pool._cache and thread._state != TERMINATE):
            pool._maintain_pool()
            time.sleep(0.1)

I understand that the delay of 100 ms is used to check regularly the stop condition changed. This sleep causes a mandatory delay of 100 ms on Pool.join().