Much of this has to do with instincts more than it has to do with size or speed.
Quiet as it's kept, Sean Spence wasn't a speed demon if you take his size into account. Sean ran a 4.7 at 230, while Olivier Vernon ran slightly slower at 260. Sean was lights out because of his instincts, not his physical profile. It's about having linebacker instincts and guys like Jermaine Grace and Juwon Young have that IN SPADES. It's about being able to diagnose a run alignment, read you keys, and choose the quickest route to get to the right hole, while also either avoiding or shedding blocks. Sean did a great job of that. Honestly, how much does running 4.5 help you when you are playing 5 feet from the line of scrimmage? It's about explosion and diagnosing a play quickly.
These arguments over size and speed are fruitless. I do know one thing, I'll take a big and slow guy, before I take a small and slow guy, because guys who have the instincts of a Sean Spence are few and far between and even they will be compromised some when going up against elite competition, because of their physical limitations. Guys like Cornelius and Armbrister and Gaines aren't BCS caliber linebackers. Hopefully guys like Fig, Owens, and Young become the norm.