docs: def rate_unstable(self):
          """Returns an unstable rate based on the last two entries in the timing data. Less intensive to compute."""
          if not self.started or self.stalled:
              return 0.0
          x1, y1 = self._timing_data[-2]
          x2, y2 = self._timing_data[-1]
          return (y2 - y1) / (x2 - x1)