There has been a 2-1/2 mile section of one of my most common routes that is (was) the absolute worst stretch of road that I've ridden. The term "rough" doesn't begin to describe it. It was rough both because of the nature of its' construction and its' condition.
The section of REALLY bad road that I would first encounter was right after what passes for a major climb around here, about 150 feet over 3/4 mile. It is a couple of hundred yards downhill at about a 2% slope and I would almost always coast down it at a gravity propelled 12 MPH. That was fine with me because to go any faster than 12 on this section of road would literally blur my vision.
It was repaved a couple of weeks ago () and I've ridden it around 5 times since. What was a 12 mph gravity only coast is now a 17 MPH gravity coast. That means that there was enough energy expended in vibrating the bike to reduce the speed by 5 MPH. I guess that if you bother to think about it, the energy necessary to rattle you about has to come from somewhere but it has to be a surprise that it has this high a velocity cost to it.