It's the mental aspect of QB play that makes or breaks them. Any one of them that manages to take the field meets the minimum threshold of physical ability. "Decision making" encompasses a lot of things, some of which are:
- command of the offensive system
- ability to develop insights from film study and then translate that to the field
- processing speed on the field, at the line and after the snap, at the NFL level (you got it or you don't)
- understanding what his receivers can and cannot do
- developing "chemistry" with those receivers which boils down to seeing the field in the same way, being on the same page in how routes are run or the option route chosen
That all has to come together inside 2.5 seconds. It's been compounded in recent decades with colleges ditching pro style offenses--they've figured out they need to simplify for youth who they have for only a short time.
Many are called, few are chosen for the NFL long term because few are able to master the mental aspects of the game. A disproportionate number of first round picks end up journeymen placeholders or backups. You willl not begin to know if you might have a winner until you put him on the field in money games. Then beware of the triumph of the uncluttered mind; promising first year starters often don't develop any further or go backward as needed refinements clutter the mind and pressure mounts to fulfill early promise.