As commuters, we're more alert to safety and surroundings than concentrating on average speed. However, cadence and speed do help indicate fitness. Of course, there's wind, weather, terrain, equipment (e.g., type bike, tires...), extra "baggage" etc.--all of which factor and make each of our commutes unique.
FWIW, mine's a mere 6 miles--some cross/tailwind and slightly downhill going to work; opposite for the return. I just replaced my tires and am seeing some gains. Going to work, I average 16.5 mph and 14.5 mph returning home. This is on-bike commuting average, excludes time stopped, but does include decel/accel to the intersection.