code: function() {
        if (progress.length) {
          SpeedIndex = 0;
          var lastTime = 0;
          var lastProgress = 0;
          for (var i = 0; i < progress.length; i++) {
            var elapsed = progress[i].tm - lastTime;
            if (elapsed > 0 && lastProgress < 1)
              SpeedIndex += (1 - lastProgress) * elapsed;
            lastTime = progress[i].tm;
            lastProgress = progress[i].progress;
          }
        } else {
          SpeedIndex = firstPaint;
        }
      }
docs: Given the visual progress information, Calculate the speed index.