I think this ties into the aforementioned reliance on frame data to estimate character viability. Heavyweights generally have poor frame data outside of a few specific moves.
I'm also not certain people have fully internalized things like the universal 3-frame jump squat and weaker shields. We're still sort of collectively in a transition mindset and relying on old Smash 4 tactics and knowledge. It'll take some time before Ultimate evolves into its own thing instead of simply "Smash 4 after a giant overhaul patch."
Facts, the shields in particular is why i've been rooting for heavies since the first time i touched the game. They can pressure you just by making you block attacks, something heavies haven't been able to do since.....well, since they buffed shield regen and removed shieldpush off the ledge from the game. In this game it's actually a real danger that your shield will pop.
I assume, with "heavyweights", we're talking about Bowser, King K. Rool, Donkey Kong, King Dedede, Ganondorf, Charizard, and Incineroar?
"Slow" could mean two things: Movement speed and frame data. If we look at movement speed we see that both Ganondorf and Incinearoar are at the bottom of the cast, making them really slow (and prone to being projectile camped). King K. Rool and King Dedede are some of the slowest characters as well (with King Dedede having the worst airspeed in the game). That leaves Bowser, Donkey Kong, and Charizard, who have fairly good movement speed.
Next is frame data. These characters tend to have strong attacks with good reach, but they tend to be slow and not combo well. Bowser's fair, for instance, comes out frame 11, same as Ike's (who is considered a slow character). Bowser's f-tilt comes out frame 10, up-tilt 11, down-tilt 10. His fastest grounded normal is jab, frame 7, which is a very slow jab (up-B and side-B frame 6).
Anyway, yes, these characters are pretty slow, and while they hit hard they have flaws keeping them from becoming top tier* (and I doubt any of them are high tier either, unless they have some hidden tech not discovered yet). These characters tend to struggle vs strong projectile characters and characters who can combo them well (they're big bodies and easy to combo), they also tend to have trouble landing.
*Charizard is a bit of an exception since he can press down-B. Charizard alone would not be that great, but together with Pokémon Trainer they create a potent character.
-- Good movement, great range, tons of kill moves, Fire Breath deals tons of damage. Very dangerous pressure on defense, almost all of his specials are shieldeaters...and he has a fast command grab that kills reliably (especially on Battlefield), and you literally cannot block Bowser Bomb or you
lose a stock.
-- Insane range, grab is
lethal, great aerials, great smash attacks. Specials also have ridiculous range, and SideB in particular is literally a shield breaker with deceptive range (if you've blocked anything minor before blocking this, you lose a stock.) Suffers from a dangerously predictable recovery, but hey it has armor on the stage.
-- command grab that leads to tech chase, high active wizard kick (kills now), great smash attacks, aerials all kill very early. Pushes a very strong advantage and punishes mistakes harder than anyone on the game.
-- again we have a heavyweight that deals a ton of damage, kills fast, and does not allow you to defend or even roll against him. Recovery is F-tier but on the stage he really only needs to catch you once or twice.
-- Probably the worst heavy on paper IMO, but he runs a really aggravating zoning game and has alot of gimmicks. Generally controls the whole pace of the match.
-- Surprisingly decent chaser with a few really oppressive normals and a Smash4 tier recovery. Probably one of the best ledge chasers in the game.
-- This character rewards good spacing so well. Again we have a command grab that kills (AND gimps), highly damaging plasma breath (gimps as well), great aerials and if you get hit by tail then you're basically in kill range and can die immediately after because you're in a tech chase.
-- Bair, downB (lol)
Most of these characters don't even need combos because by the time you're at 45% you're getting launched far enough to get edge guarded, and around 60% you're in danger of being killed. Anything past about 95% and now we're talking about you getting kills from trades, and without gimping these characters can easily live until the 150-160% range.
Projectiles are a problem for them (well they're an issue for everyone on this game) but most projectile heavy characters on this game suffer from a common issue, they have trash recoveries and are generally pretty light, so the tables turn on them very fast.
I don't have enough hands to count the number of players I've seen be inspired to play robin, only to drop him quickly due to how different he is from almost all the other characters.
