Returns an unstable rate based on the last two entries in the timing data. Less intensive to compute.

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)