Not for me it isn't, but that's not to say you couldn't do it.
My commuter is a tank, my commute has a bunch of hills, and I'm not in good enough shape to bust out with a 16mi time-trial twice a day.
When I lived closer to work (7mi one-way) and rode a flat path most of the way to work (no lights, no crossings, no stops) and just 1 hill afterwards, I could hit 18mph average if I was really pushing it.