Changed from a 6-step slider to a 10-step slider.
Speed factor from
{ 0, 0.2, 0.5, 1, 2, 4, 20 }
to
{ 0, 0.2, 0.5, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, 4 }
20 is too slow and isn't very useful. So I dropped it. 4 is a good maximium value: it looks very slow but still acceptable.
Between 1 to 4, here are 5 more ticks, so users can control more detailed speed. It is
the range that makes most difference.