It appears that the copy dialog displays the average copy speed. This means that over time for any large job it will likely become a flat line like the image below. Even if the current speed significantly drops you are unlikely to see a change the to average for some time.
I think it would be more informative to see the current speed as the green bar, with perhaps the average and or peak as a single lines over the top.
Also it appears as the graph does not go all the way to the edge of the image, not sure if there is a reason for that.