I get that this isn't necessarily exactly applicable, but look at the careers of Rodgers vs Luck.

QBR through first five years as starter:

Luck
76.5
87.0
96.5
74.9 (only played 7 games)
98.3

Rodgers
93.8
103.2
101.2
122.5
108.0

Both were highly qualified QBs coming out of college, with Luck arguably being the better prospect. They'd both had long, successful college careers and "had seen a lot of defenses." One was given the reigns to a brand new offense on day 1 and there were growing pains. A lot of 'em. The other got to sit and watch for a couple years, figure it all out, and step in pretty seamlessly. I'm not saying that Taysom is Brett Favre or that Tanner will be Aaron Rodgers. Nor am I saying that either one is Andrew Luck. But there's certainly something to be said for taking some time, getting a feel for a new system (not to mention level of play) and allowing the particularities of an offense become habitual and muscle memory instead of driven by conscious thought.