Speed really does vary. On my recent tour, I found a certain 50 km stretch especially grueling. It took me five hours, and afterwards, I was wiped physically and mentally. I spent the next two days resting.
Yet I have an acquaintance who completed the same route in two hours, and was fresh as a daisy.
The differences? There are three that I know about...
1. It was my fourth straight day of hard riding, and in retrospect, should have taken the day off.
My acquaintance was on the last leg of a two day trip.
2. My acquaintance had tail winds most of the way, while I faced headwinds.
3. My acquaintance was 28 years old, and I am 53.