Geez, tricycles can go 10mph.
Normal riders go 12 to 15mph. Someone with a normal road bike easily can average 20-24mph.
I average 25-27 on a 50 mile ride.
Racers can exceed 40 average in a peloton.
My comment is not scientific, you are correct. It is experiential — when I drive along the road and there are bicyclists and I am going 30, it SEEMS like they are going 10 mph. I might just lump 12-15 mph in with ten, and say ‘It SEEMS like they are going slow.’