The issue with speed is what are we talking about? Speed in attacking or movement. Ike in both Path of Radiance and Radiant Dawn would be more like fast in attacking which would be insane in SSB. Could you imagine him swinging Ragnell faster than Marth or hitting as fast as Captain Falcon? Ike and his father, Greil, had a unique fighting style and they were known for using longswords and two-handed swords with one hand. He could be slow while running and stuff, but if he could cut you down in a heartbeat, it wouldn't matter as much. Ike having good air speed is fine since momentum reasons, but running faster than Falco doesn't. Falco should be faster because he's more lightweight while Ike's lugging around a two-handed sword and armor.
Falco's on the other spectrum, similar to Dr. Mario, where he's slow in movement, but he hits fast. For balancing reasons, he should be faster than say, Pit - I think Pit's dash speed is fairly fast. He should be that slow in running, though. At least he walks fast and his short, "fast" initial dash makes his foxtrots good.
Ganondorf sharing Captain Falcon's aerials except Fair ends up being fairly fast for his class of heavyweights and he's hella strong as most people know. Imagine him with Bowser's dash speed. Or even give him his Melee dash speed. He would rise up in tiers because he would not only have strong attacks and fairly fast moves, but now he can rush in or even outspeed people.
It's mostly balancing since if Ike was like a Final Fantasy/Kingdom Hearts character, he'd make Brawl Meta Knight look balanced. Or if Ganondorf went Vegeta on everyone, he'd make DK with Kong Cyclone look like Yamcha. There are just some issues, though, like Falco running slower than Ike while in Brawl, I think Falco ran faster than Ike, but not by much. Samus could run faster, but it's not really a problem.