Roy has a higher falling speed, creating a much stronger SHFFL. While his speed for attacks and dashing is by technicality standards slower, it's very minimal almost to the point of being irrelevant. For contrast, his power is by technicality standards lower then Marth's.
I thought this was an interesting claim, particularly the part claiming that Marth's speed advantages are minimal to the point of being irrelevant because that didn't match my experiences. Here's what Marth has over Roy (IASA means "interruptable as soon as" and is the time it takes to be generally free to act, not the same as the mostly irrelevant animation length):
Ftilt hits 2 frames faster and is IASA 5 frames faster.
Dtilt hits 1 frame faster and allows shield 8 frames faster (IASA for non-shield actions is identical)
Utilt hits 1 frame faster and is IASA 8 frames faster
Dash attack has essentially identifical frame data
Fsmash hits 2 frames faster and is IASA 6 frames faster
Usmash hits 2 frames faster (same IASA)
Dsmash hits 1 frame faster with first hit, 3 frames faster with second hit, and is IASA 10 frames faster
Nair hits 1 frame faster with first hit, 2 frames faster with second hit, is IASA 1 frame faster if finished in the air, and has 3 fewer frames of lag with l-cancel
Uair is IASA 4 frames faster if finished in the air and has 2 fewer frames of lag with l-cancel (same hit frame)
Bair hits 1 frame faster and is IASA 8 frames faster if finished in the air (same landing lag with l-cancel)
Dair hits 1 frame faster and is IASA 5 frames faster if finished in the air (same landing lag with l-cancel)
Fair hits 1 frame faster, is IASA 5 frames faster if finished in the air, and has 3 fewer frames of landing lag with l-cancel
Counter begins active frames 3 frames faster (same total commitment on whiff)
Dancing Blade has essentially identical frame data between the two of them (some obvious variation when they actually do different attacks)
Neutral special has essentially identical frame data other than that Roy's takes a lot longer to charge.
Dolphin Slash hits 4 frames faster and lasts 9 fewer frames. Roy has his literal only relevant point of frame data advantage here though as his landing lag from fall special is 4 frames faster.
Grabs are identical.
Rolls and dodges are identical.
Marth leaves the ground 1 frame faster when jumping.
Funny thing, before I started going through it, I wasn't quite sure just how much worse Roy is than Marth other than intuitively "a lot". Wow, Roy is bad, especially when it comes to the recovery times of things which is doubly bad when you consider that Roy's hit properties punish him for spacing stuff fully so you end up with this laggy stuff that only hits at full power if you space so badly you'll get punished on block for sure. Something like SHFFL fair even doesn't really go Roy's way. Marth leaves the ground one frame faster and hs a fair that starts up 1 frame faster so you're hitting 2 frames faster to start if you choose to attack at max speed (which you don't necessarily have to), and then Marth recovers 3 frames faster from the overall action with the l-cancel which has to make up for any possible advantage from Roy's falling speed and really is even better since you can always just do the aerial later if you want more safety on Marth but there's nothing Roy can do to be that safe.
Apparently Roy actually does have a 7 frame faster SHFF time which I do have to admit is nice (only 6 frame advantage if you consider Marth's faster pre-jump animation), but in terms of overall speed that's definitely a less big deal than Marth's mountain of other advantages. That fall speed in the context of the overall game is even pretty debatable in terms of merit. On the plus side it gives that nice SHFF for approaching grounded opponents and makes him more resiliant to vertical KOs; it's definitely better against spacies on those grounds at least. On the minus side, it makes him fully cg'd by Sheik, makes his recovery worse, and hurts his ability to air to air which is definitely the more relevant factor against Peach and Jigglypuff so that's one general disadvantage and three top tier match-ups that it's a definite downside in. I'm not familiar enough with Melee ICs or Falcon to know what kinds of combos they have that vary between Marth and Roy based on the fall speed difference, but that would be the next point to investigate if you were super interested in breaking down just how good or bad Roy's fall speed is for him.
Well, that was a fun way to spend a half an hour. I sure hope no character is as outclassed in smash 4 as poor Roy...