I'd argue that it is Larsson. His top end speed isn't great, but he is very agile on his skates for his size.
I feel like we've never even seen where Larsson's top end speed could be (at least not in-game). From what I've seen/can remember, I feel like he's barely taken on the role (or been given the opportunity) of being the guy actually skating the puck up the ice/taking the zone. For the most part, he's been a guy making the breakout pass, or in some cases being the defenseman trailing on the play. I don't think his top end speed is exactly earth shattering, but most of the speed we have seen from him has been a product of his stride and overall agility/mobility. And we haven't seen much of him having to sprint back because he got terribly beat on a play going towards his own net (which isn't a bad thing).
If he was on the PP, perhaps say in the role of Zidlicky, I think we would get a better feel of what his speed really is. Again, probably not anything too amazing, but I think he could be a little faster than the way he has played.