Wait, what? I bike commuted for a while in both Cleveland and Boston. I can pretty handily maintain 30kph, and regularly hit over that when either warranted by traffic conditions or on a slight downhill.
I do not consider myself a fast cyclist by any measure.
I am quite confident that even the most casual group ride of semi serious cyclists would drop me in 5 km on a good day.
Is my view of cycling speeds that skewed, or is the distribution very weird in some way?
As for the extra 5kph making a difference, no it's not a huge deal in time. It's immensely important to riding safely in faster traffic. I find that the closer you can ride to the speed of traffic, the fewer boneheaded things people do to get past you.
Being able to do 40kph for short stretches to basically match the speed of cars through a dodgy intersection is a huge improvement in safety. 30 isn't 40, but every little bit helps.