Metaphor (if it is really a metaphor) is OK for me. I always understood it this way: time on X, stuff to learn (not some performance) on Y. Somewhere on that Y

Metaphor (if it is really a metaphor) is OK for me. I always understood it this way: time on X, stuff to learn (not some performance) on Y. Somewhere on that Y is the threshold point and by passing it you cross from "still learning" to "doing actual work" area. I admit, time on X may look ambiguous but I kind of look at it as "some reasonable time interval". That way something complex will require to learn more going from 0 to threshold point and hence steeper curve (line) than simpler stuff.