In Moonwalking With Einstein, Josh Foer discusses something called the “O.K. Plateau”:
In the 1960s, the psychologists Paul Fitts and Michael Posner tried to answer this question by describing the three stages of acquiring a new skill. During the first phase, known as the cognitive phase, we intellectualize the task and discover new strategies to accomplish it more proficiently. During the second, the associative phase, we concentrate less, making fewer major errors, and become more efficient. Finally we reach what Fitts and Posner called the autonomous phase, when we’re as good as we need to be at the task and we basically run on autopilot.
The idea is essentially that we rise to the level where we are “good enough” at something and then we tend to slack off on it. For example, most of us get to a comfortable speed with typing–usually trhough some sort of structured program+whatever we feel we need for a given task (between 30 and 80 WPM even for professional typists)–and then we stop improving. In large part because we stop practicing against an improving standard. When people actually focus and work at it, comparing themselves to others or to slightly accelerated versions of their own records, they have a tendency to start improving again and get significantly better.