Looking at the major factors of BB, lets say Training, Diet, Recovery, the answer must lie in recovery, right? If someone gains muscle, it's because they recovered. If they gain it fast, it's because they've recovered fast. Muscle doesn't grow unless it's recovered enough prior to new stress.
So then...nervous system recovery or tissue recovery? Both?
My big guess is that nervous system recovery is the most important. Everyone taking drugs, eating and sleeping is recovering faster. But few of us are freaks. Nor are we that unGodly strong that many freaks are.
But what if we could TRULY, fully recover. Fast. We could hit it again. Every time. Train, recover, increase the weights, increase the reps per set.
Look at how strong a lot of the freaks are. That's a superior nervous system and tendon strength. Levrone (and I always think of Troponin too, strong as an ox and massive to go with it.) So, my vote goes to better nervous system recovery in a freak. Probably goes right along with better muscle recovery too.