Ratings for grade and time are all so subjective that they are to a degree BS.
First, one can ask how the guy did the rating systems in 1963. There is a lot of misunderstanding because people don't take the time to read and explain what the other did, before being able to decide what is good or what is bad. Ordinarly, people try to keep there idea even if they don't know the other theory.
For your knowledge only, we can describe the rating objectively. If you take one hundred climber and bring them to new cliff to climb route that they never heard about it, you are going to be able to do statistic. It is what people call a common agreement I think. But it is measurable. In fact, each person as there limit. One climber do all 5.6, 70% of the 5.7, 50 % of the 5.8, 30% of the 5.9 and 0 5.10. it is a normal curve. With those data, you can say the level of the climber as he climbed 50% of the route easily and he didn't know for the other 50% if he is good enougth. If you take all the climber who climb 50 % of the 5.8 route, you can analyse the difficulty of the route that they climb and describe objectively the limits of those climber, as most of the time, climber fall at the same place. It look complicate, but there is many complicated things that we can oversimplify for convenience. For example, the rope. all the theory about the rope is very sophisticates, but we can resume it with the fall factor for all climber.
Statistically, one person could compare his performance with the one of the population by climbing on sight a route of the same grade in an other area, without asking all the beta to avoid any problem and climb one or two grade over his limits.