This matches my experience on shared pedestrian / bicycles low density roads in countryside. 30 km/h (a little less than 20 mph) is the limit between okish and dangerous. Actually pedestrians start to worry a little before that speed but of course half of them don't see the bicycle approaching. The cyclist see them a long time before overtaking and assesses whether they are safe to pass or require a slow down.

This is a good example of how we get bad laws... Should overweight cyclists be forced to have a lower speed? I can still get my ebike to 25 mph down hill, I would never dream of passing a pedestrian at that speed. I picked 24.5 for a reason and not 25 BTW.