Thank you, Antvasima. Very much.
Okay, I realise the consensus seems to be that the flight feat is an outlier because later feats contradict it.
AMM uses this feat as an example:
https://youtu.be/dUvCvBdYrkA?t=3m48s
It certainly seems like he's flying slow there, but as you can see regardless of how fast he may or may not have been moving, his speed in that scene was clearly sufficient for the task at hand; he and the injured Lucia escaped unscathed from the explosion. There is also the fact he is holding Lucia who is weak at that point, and might not withstand the force that would hit her if he flies full speed while holding her, meaning he has no reason to fly any faster than he did, and might have a good reason to slow himself down. This isn't unusual as Dante always holds back on his true abilities.
Basically the issue with the feat is nothing else supporting it. For what it's worth there's this clearly unquantifiable feat which could scale very high:
https://m.youtube.com/watch?v=xx257VkmCEo
This is Dante vs Argosax. The speed feat starts at 00:49. As you can see Dante is casually speedblitzing a demon considered superior to Mundus and Sparda, which is supported by the way it's stated that at this point Dante surpassed "all who came before him". A totally unquantifiable feat but via scaling it is relevant.
And for his generally lower showings, Dante is extremely casual when he fights, as shown here:
https://m.youtube.com/watch?v=kV-Z19chIJQ
That is a lower level demon tagging Dante, but he deliberately lets her tag him simply because he knows she can't hurt him and wants to troll her, and she's actually stronger than most demons he fights.
This is repeated in a different way here:
https://m.youtube.com/watch?v=qPMNWFmD5Qw
Dante states here that he wants to know what Baul is like as a fighter, and deliberately lets Baul tag him so he can hit him back, as shown by his laughing at Baul right after Baul tags him. Dante is enjoying this fight for the swordsmanship of his foe, and is noticeably drawing the fight out so he can enjoy it more; in this video you can see their strikes have roughly equal power while Baul is in human form and yet when Baul goes berserk and transforms Dante casually overpowers him even though Baul is stronger than before. Remember Baul is extremely powerful but is also one of the best swordsmen in the world, hence the enjoyment. And since Baul is high level we know he's one of the faster demons.
In short Dante holds back immensely and deliberately allows enemies to tag him, even holding back to closer to their level so as to enjoy the fight more. Considering how casual, even deliberately reckless, these feats are it seems unfair to use them to discredit the feats he performed the one time he fought seriously, especially since many are guilty of being unquantifiable rather than sound low end feats, and even when being so casual he still stomps most of his enemies.