The sophomore slump is this mythical beast: the perpetual fear that a successful rookie will collapse for completely unidentifiable reasons. Sometimes, that does happen -- as happens with players at any stage of their career, every season. But there's little evidence that the sophomore slump is a thing, and lots of evidence that players consistently get slightly better between their first and second seasons.
This is also true of quarterbacks, unsurprisingly. Of 18 first-round rookie quarterbacks to have started at least eight games since 2006, 14 improved in their second year, according to SB Nation's Adam Stites.