Locks are all about minimizing downtime, and blowing through multiple mobs at once. Once you hit 30 and get siphon life (assuming you went affliction up to that point) theres no downtime unless your pulling 3-4 mobs at a time with no stop for long periods of time. In my opinion affliciton is the most efficient leveling spec, although 5 boxing felguards will kill mobs really fast, in fact your gonna want to make sure you hit the mobs before the felguards steal the kill.