There are other starters who are much faster, but with no reason. And since Incineroar is also a starter, I don't get it why the rules don't count the same with the others for Incineroar.
Venusaur(80): Thick Fat as it mega evolves, weighs 100 kg (342.8 lbs) (mega has 155.5 kg (342.8 lbs), has a tree on it's back, doesn't look too healthy, heavy moves(like body slam)
Emboar(65): Doesn't look healthy, short feet, heavy moves(like heavy slam), 150.0 kg(330.7 lbs), is a pig (:P)
(although it's only five speed points over)
Incineroar(60): long legs, wrestler, 83.0 kg (183.0 lbs), learns Fake Out, Incineroar's signature move is literally spinning around really fast with your hands in fire(in the anime it's literally a tornado), it's belly stripes are mostly based of a six-pack if it has muscles there it should have muscles in it's feet making it even faster
For some people who think Fake Out and being a wrestler doesn't make you fast:
I personally think being is pretty much equal to attacking fast. Since Venusaur is actually a special attacker, so it doesn't need to attack fast with it's legs at all. Probably that's why it has mediocre speed. And a wrestler needs to damage the opponent as fast as it can, mostly without running hard. Just jumping aside. And Incineroar seems to be a good wrestler (that attack stat of 115 is great for a starter). And if it wasn't fast, why could it jump to the opposing trainer that it's battling? And Fake Out for a non-fast Pokémon doesn't make pretty much sense.
Emboar maybe could hit fast too? But I still don't believe it for Emboar, since it's arms may be too muscular and then weigh too much to hit very fast. and it couldn't kick with these short feet as well. How is thing even faster then Incineroar?! Incineroar could do both, jump, and kick. Incineroar could even bite hard, since it evolutionary line has bite. And yeah, Emboar doesn't. Heck, Incineroar could attack with everything, it even has sharp claws and it's evolutionary line learns scratch.
Well, Venusaur actually makes sense, because it is an special attacker, and special attacks go about how fast you recharge. But when Venusaur Mega Evolves it becomes mixed, and it doesn't make sense that there is still one speed stat for both of them. Since both attack in a different way. But yeah GameFreak does never make sense :/
Well, I have being theorizing a lot, so I might have gone wrong horribly on one thing, but still, even without my theorizing Incineroar is long, and has long legs. I hope someone can find this out! You can use even your own theory that's completely else then mine to answer. And if you do, thanks alot! :)