Two cars are driving from A to B. Ideally they want to get there as quickly and efficiently as possible, without driving dangerous or stressing the cars. Car 1

Two cars are driving from A to B. Ideally they want to get there as quickly and efficiently as possible, without driving dangerous or stressing the cars. Car 1 takes 15 hours to drive from A to B. Car 2 takes 10 hours. Why is that? Well, car 1 has a 1.3l engine and took a wrong turn because the navigator can't read a map. Car 2 stopped 3 hours in because it had a flat, but they got going again much faster. The speedometer won't tell you any of those things. It can't tell you WHY you're going slowly (you're under-resourced or you have a problem that needs fixing) or if you're going the wrong direction. That doesn't mean it's useless. It can still tell you if there's a problem, or if you appear to be doing well but you're not making any progress, maybe you're going the wrong way.