20 mph (32 km/h) on a regular bike is doable, but yeah, usually that involves a very "flat" road or even a road that has a slight decline. And as you've said, maintaining it (e.g. for more than 10 seconds) is a whole different story.

Furthermore, it also requires a certain fitness level and "bodily involvement". The thing that still catches me off guard at times is how relaxed some people on ebikes look while going that fast. Whatever kind of judgement I could make in the past on how fast someone is approaching based on how much they "visually excert themselves" (e.g. hunching forward or even standing up) kind of has become meaningless with ebikes.