What's interesting, however, is that throws that occur after a longer period of time after the snap are associated with such a lower QB rating. One would think that a QB, if given time, would have a better QB rating, even if the primary receiver isn't open on the play. However, what seems to be the case is that the extra time to throw creates a much poorer outcome with regard to QB rating than does having an open target earlier in the play.
In other words, one would think time drives the bus on QB rating in this regard (i.e., QBs perform better when they have more time to throw), but what the data suggest on the other hand, IMO, is what you said, that an open target earlier in the play does a hell of a lot more than time for a QB's performance.
That is an interesting finding IMO, and it flies in the face of some of the intuitions I've seen here that sound something like, "I bet Ryan Tannehill's QB rating is a lot better when he has [or takes] more time to throw." In fact Tanenhill's QB rating is six points lower on throws he's made at 2.6 seconds after the snap and beyond, which is no different from what happens across the league, for almost all QBs.