Well actually no that's not quite right, you need to be faster than **everyone** you are chasing or who is chasing you, and then you need to be able to tackle or avoid the tackle when/if you get there, or choose the right pass, or turn and kick chase etc.
For example, last Sunday Meaney scoops up a bad pass and runs off, with 3 in support. Benji and Mbye very easily cover the distance with enough time to effect a tackle, but they are both running with their backs turned, they turn the same way and suddenly Meaney has drawn Mbye and passed to the outside. Benji attempts to affect a tackle but is bumped off, Bulldogs score their first try.
Now is that something to do with speed, or positioning, or judgement, or defensive ability, or all of the above?
In fact I didn't see any try conceded, or any potential try not scored simply because the Tigers players weren't fast enough. Not committed enough certainly; also not fit or aware enough - all of those, but no obvious situation where a JAC-type scores.
On the other hand, there are many situations during a season where "JAC scores that" can be said, which is dandy, because there's only 2 or 3 footballers in the competition in that speed class.
Also, what you've said about "I'd say that X players are faster than Y players" - well frankly that's all guesswork to the extent of being pointless.
There is no way to measure speed of entire backlines across the competition, then you are asking "What kind of speed do you mean? Off the mark, middle distance, long distance, with/without football" - to such an extent as it is meaningless.
Does Will Chambers outpace Marsters and Fonua if he's in the clear? Maybe. Does he also outpace the fullback and any cover defenders? Who can say in advance of any individual play.
Will JAC outrun most footballers in open space? Yes. Will James Roberts. Yes. Will anyone else - hard to say. I've seen Ben Smith run down Benji Marshall, I've seen Simon Mannering outpace Chris Lawrence, I've seen Tui Lolohea mow down Ben Barba like he was in quicksand.
So yes buy the fastest and strongest and best players you can. But specifically are Tigers not scoring because we don't have enough fast players? I'd say that's not our major concern right now, our concern is consistent intensity and application of the coach's plan every single week.
Tigers are currently 4th on table, 4th-best F/A, 3rd-best defence, 8th best attack.