If you commute you will mostly ride on dedicated bike lanes without many stops so you can go at full speed most of the way. I've commuted around 3 miles in a not so bike friendly city in Europe and it took about 15 minutes since there are dedicated bikeways without any traffic lights you can take most of the way.

If you commute in rush hour that is actually a lot faster than going by car, so biking is the fastest option available.

Edit: And that was at a leisurely pace, similar to the effort of walking.