In my experience the danger zone when cycling on a mixed traffic bike path (pedestrians, slow bikes, etc) starts at 25 km/h, which is about the double of the speed of the casual or low range cyclist. 20 mph (32 km/h) is too fast for the other traffic. They don't expect to have so little time to react and get scared (even if the cyclist does everything and saw them since long before, they only have to keep doing what they were doing). Furthermore with a normal bike it's a waste of energy so when traffic increases I slow down because the average speed doesn't change much if I sprint between each group of pedestrians/slow bikes. I understand that with an engine you can do a lot more of stop and go but please don't scare people too much, it makes them hate all cyclists.