Or maybe you are slow...

Not to brag, but as an example, when my CPU says I averaged 18.5 mph or so I am making my 17 mile commute in about 55 min. That includes lights, which there aren't tons of, but a few, and also includes a good hill where I'm usually only pushing 13 mph. I make up for it on the long, open flat sections by rolling at 22-24mph.

Oh yeah, last thing...I was off the bike intermittently over the last month or so, and tried to push it like that last week. Hurt. I still managed 60 min., averaging 17 mph, but it REALLY hurt, and I needed a few days to recover. My top end like above was in best shape this summer, and no wind.