Time is the measuring stick for great QBs. What often separates good QBs from average QBs is the time they need to make decisions and find throws (see also: Hendon Hooker). The greatest QBs don't need a lot of time. Good QBs can play great if they have a little more time - (think of teams with superior OL who win big with "game manager" type QBs). And average QBs can often look pretty good if they have enough time.
If you don't have dominant DL talent that can beat OLs and get to the QB, or even if they're dominant but they just can't get to the QB at the end, then the QB is going to have all the time in the world to find someone to throw to. Then you're throwing to WRs who have room to catch short passes ... so it's all on the DBs to play perfectly and bend but never break.
2006, JaMarcus Russell threw three picks under pressure in Knoxville; the book on him before that game was that he made bad decisions under pressure, so big shock there. End of game, Tennessee with a small lead, and the Vols dropped back into those giant cushions instead of pressuring Russell ... and Russell found a guy and made the throw to win.
2008, Norm Chow said after UCLA came back to beat Tennessee that they were never worried at the end of regulation, and their staff had discussed that if they were within a score at the end they knew they could drive it down the field by playing a short passing game. Straight up said it on the radio. UCLA knew Tennessee would drop into that deep cushion in the secondary, and they used passes underneath to carve Tennessee apart.
2001, UGA, David Greene, well, anyone who was there had that game seared into their brains for all time. Lot of time, lots of room.