Processing speed is a trained skill
The hardest adjustment for a young quarterback is rarely arm talent; it is recognising, in under three seconds, what a disguised defence is actually doing. That recognition is built through repetition against speeds and disguises that simply do not exist below the professional level, which means a rookie is being asked to learn a new language while also being tested in it live.
Some players arrive further along that curve than others because of the system they played in before, but almost none arrive finished. The plays that look like poor decisions in October are frequently the same plays a player will read correctly by his third season, once the picture has slowed down for him.
The supporting cast shapes the evidence
A rookie season is not a clean experiment. The offensive line's protection, the receivers' ability to separate, and the play-calling around him all shape the raw statistics attributed to the quarterback, and a poor surrounding roster can make an improving player look stagnant on paper.
This cuts both ways: a strong supporting cast can also flatter a passer who is not yet processing the game well, which is part of why evaluations that lean only on box scores from a rookie year are unreliable in either direction.
Two clocks running at different speeds
Coaching staffs and front offices generally plan development on a multi-year horizon, because they have seen the pattern before and know what the middle of it looks like. Public reaction, driven by weekly results, runs on a much shorter clock and rarely tolerates the plateau that often comes before a real jump.
The useful discipline for a viewer is treating a rookie season as an early data point rather than a verdict, and watching specifically for the speed of decision-making improving over the year rather than the counting statistics. Arena's NFL positions guide describes what the job actually demands before it demands winning.