Not the case. In fact he was quicker than any of the All Blacks that played in 2010 according to the EoYT speed tests, at least over 40m. He'd be a hell of a lot quicker than Ranger. If you watched Blues v Hurricanes this week, he absolutely burned Hosea Gear to dive on the ball, and pass it. He's still very, very quick, and he's very strong. Physically I'd say he is the best winger in terms of speed and size apart from maybe Gear. His problem is with how he is playing the game. Count how many rucks Rokocoko finds himself in compared to Rene Ranger. Rokocoko is all over the place, which means he will rarely be where he needs to be in space. When he does get in a bit of space, he doesn't seems to hit the gas, but instead tries a little side step.
He's young enough to have a change of form, and even another potential World Cup, but I doubt he'll stick around for either. I can't find where he burns Gear in the highlights, but look how quick Rokocoko is moving to get to Cruden before Hosea Gear's try: 1:55
And he almost got to his feet to try and chase down Gear, but slipped.