For anyone to be a universal threat to
with such urgency, they need to be at least FTL AND preferably be able to survive the vacuum of space.
We know Freeza “has” to be FTL for the simple reason that he wasn’t afraid of blowing up Namek, and we know he wasn’t planning on traveling to the next solar system for next 1000+~~~ years.
I also think we’re being disingenuous with
early DB being relativistic in on-panel feats like instantly destroying the moon,
and characters in early DBZ far outclassing them in speed.
We also have Goku dodging solar flare in
23rd Budokai, he was quick enough to dodge and steal Roshis sunglasses.
A similar case can be made for Cell and Buu.
No one would care if they could wipe out the
universe if it’ll take them billions of years to
get to wherever they are.
A more natural threat is likely to kill them
all before Cell/Buu could reach them.
I don’t think I need to even bring up
RSSJ Broly who destroyed most of south galaxy in what is implied to be a very short time.
Are those characters even referred to as a threat to the Universe? Or just as a threat to the world? Besides, I don't think Universe needs to be taken literally. It can just mean a threat in the sense that no one will ever be able to stop them wherever they go, rather than traveling as far as they can. For example, they couldn't reach the Kaioshin Kai without the Shunkan Idou, since it's said in the Daizenshuu the only way to travel there was through teleportation.
Also, I know the attacks are implied to be that fast... Which is why I think the characters can react to FTL attacks in a quick way, but can't really move through long distances. Goku also spent months traveling the snake road which was 1,000,000 km... And Goku also took some minutes to get to where Vegeta and Nappa was using his Kintoun to conserve energy, while if FTL, wouldn't it be sensible for him to just reach there instantly in an eye blink? That's why I think there's a clear disconnection in the story between how far the character move through flying and how fast they react to attacks quickly, the latter which is likely ftl, the former which has many examples showing it isn't.
With Shunkan Idou, Cell could also arguably be a bigger universal threat.
Also, I wouldn't exactly equate escaping from the planet as necessarily faster than light as the speed at which an explosion expands can vary greatly depending on the type of explosion and the materials involved, but it's typically far below the speed of light. I'd say considering more clear cut scenes like the snake road which are more literal, Goku avoiding the Taiyoken as far back as the 22nd Budokai intended to showcase Goku's incredible speed and reflexes in a visually compelling way, rather than making a statement about his ability to move faster than light in a literal, physical sense, much the same way I think Roshi destroying the moon can have some artistic and funny liberty taken when the moon is a huge feat, while Tenshinhan's Kikoho, said by Roshi as easily surpassing the KMHMH, created a crater where the arena was and the destruction was limited to that, but with Roshi looking at the destruction of the arena as proving how above the KMHMH it was... Piccolo's feat of blowing a city wouldn't even seem that impressive with Roshi from the 21st Budokai, even while buffed, destroying the moon, which I think reflects the earlier sagas more gag-ish and not so literal to be taken feats, imo.