I don't think it's speed alone.
Oregon's got top tier speed especially at RB.
The SEC somehow comes up with these fatties that are humongous yet very fast.
Other teams around the country might have one of these types but a top SEC team will be lining up 3 or 4 of these guys against you.

So it's line play where they really excel and of course overall defense as seen in the LSU - Bama game.

PAC 12 needs to find/recruit these big, fast fatties and then you'll see a juggernaut. Our offenses will take care of themselves.