Speed is just the best quality to have in this game because you will win the neutral game off of it every time if you use it properly.
This isn't true AT ALL. If it were, you would see Falcon and Fox dominating neutral game vs. everyone else. Obviously speed is a huge factor in determining neutral game potential, but there are plenty of others such as projectiles, range, priority, risk/safety with trades, moves having good/bad hitboxes for approaching/zoning, moves having good startup/low lag, and tons of others.
While Falcon and Fox have top movement speeds to work with (both horizontally and vertically), Falcon suffers from a lack of useful approaches. His aerials either take too long to come out or are easily CCed. His ground moves are trash in general, but are especially bad when trying to approach with them (small WD compounds the issue). Raptor Boost is the only special he could conceivably approach with, and it's certainly not a reliable one. Fox has much better moves for approaching (nair is quick with decent followups at a variety of percents, dair beats CCs), but they all have awful priority and his low weight combined with his high gravity make him super susceptible to trades.
Peach is definitely a slow character, both horizontally and vertically, but she makes up for this with a good projectile and good zoning/trading tactics. You can't just ignore what happens after neutral if you want to analyze the game. Fox may beat her in the neutral game 2/3 times, but she doesn't get punished very hard and punishes Fox super hard for a single mistake, so only winning the neutral game 1/3rd of the time is plenty. Ganon is definitely in the camp of not getting punished hard and punishing hard when he gets his turn. It's not like anyone thinks Ganon is amazing in the neutral game, and I'm sure Kage will attest to that. The question is whether all of Ganon's other good traits outweigh his deplorable speed.
Can he easily chase people onto plats? No, but his uair sure is amazing and screwing people over if they try to platform camp. Can he easily weave in and out of other characters' ranges? No, but he can hold his own zone better than almost any character, which gives him a really good chokepoint game. Zoning and earning more per hit than your opponent is the only way slower characters can keep up with faster characters in this game (and probably others tbh), so if Ganon was not built that way, I could totally see why you'd attribute his badness to being slow. Kirby is a good example of a character that is slow and bad. Is he bad because he is slow? Well, if he was faster, he would definitely be better, but he'd also be a lot better if he stayed slow and instead was able to zone like crazy. His low weight, poor recovery, and weak punishes are just as much to blame for his awfulness, if not more than his lack of speed.
I'm not saying I think Ganon is amazing and no one realizes it. I just think he's vastly underrated because people want to Theory Bros. their way around good zoning with dumb generalizations like "fast characters can win the neutral game every time if they use it properly". That's just... NOT true. You don't need to look any further than Armada's Peach to realize slow characters can be just as deadly as fast ones as a result of all of their other traits and the effect those traits have on the game.