All speeds are good, and on a commute it depends on the features of the route, mainly slows and stops. I made a private Strava segment just to track commuting time - it has me at 18 mph average, easy route. Nice weather, a little slower, miserable weather a little faster, because I'm cautious with pedestrians and other cyclists, so it has more to do with what's out there than with how fast we are.

It also helps my perspective to realize that for commuting time it doesn't really matter. Slowest to fastest this year, not counting the ice storm, was a difference of only 2.5 minutes.