think about it though, it had to happen that the power went to **** because otherwise, sparda would have obliterated Dante, since Dante was nearly defeated by his brother who had only half the guys power, and not even the sword in it's actually helpful form plus the tiny amount he had, mundus almost killed Dante with 3 spears in dmc1, once Dante went SPARDAAAAA! they don't do nearly as much, even on DMD. not only that, but this was when Dante was even stronger, as his power level rises immensely per game, this is why his regular brother killed him in their first fight and was a tough match in his second and third fight, while nelo angelo, a form of vergil that was buffed my mundus, only deserved to be the second boss you encounter, and he's not even that hard.
Dante, so to say becomes far stronger, and nelo angelos slowness may not even be a real thing either.
consider this, vergil's attacks with yamato are officially revealed to be "faster then the eye can see" i can see vergils sword moving, Dante's own power may actually slow time for himself, this is why enemies are actually stupidly lacking in aggression in dmc2, they are actually the same, Dante is moving so fast to him it seems like a joke, remember how quickly he reacted to that random explosion in dmc2? he had moved and gotten there in less time then it took me or even lucia, an experienced hunter, but much weaker demon, to know what had just happened.
the saviour in DMC4 is a slow piece of ****, yet it's powered by a guy who does not seem to be very slow, it could be the size and wieght, but science rarely applies in devil may cry, the statue is actually probably a fast enemy when worked up, he was just lazily cruisin' during the cutscenes where there was no threat, and yet, seemed faster when fighting Dante in cutscene, but not when you were playing as him.
furthermore, consider this, vergils darkslayer "teleports" show a thin streak going to the target, implying massive speed gain instant of instant reloction, when Dante uses his trickster teleport, the same thing could be in effect, he just increases his own speed skills by a massive amount.
consider how the stronger demons are, the more aggressive they are, some, of course, are naturally faster then others, but what's stopping the basic hell enemies from making short shouts before teleports or slow moving scythe attacks faster, for the scythes, it could be strength, buy demons are generally stronger then humans and those scythes don't look stupidly heavy, and why such a long yell before the teleport on the white guys? can't they just, you know, teleport?
it could be that the yells, are actually tiny quick sounds, but Dante slows it the hell down.
in fact, consider quicksilver, it's not geryons version because, Dante's functions differently (he just clicks fingers, geryon has to catch you with his orbs.) it lasts longer, and most strangely, while this may work the same as on geryon, it uses his demon energy, the source of energy that Dante loses and gains back again frequently, the resource for channeling his powers, Does it not make sense for the devil power to be channeled through and magnified by quicksilver rather then just giving him the ability.
even more convincing, is how geryon actually does not own the power himself, he got it from demonic essence, which allowed him to absorb the demon power, if demon essence overdoses equals slow time ability, is it not implied demons can manipulate time?
i think any time in the cutscenes not seen through Dantes eyes are actually moving at normal speed, consider how the only human that can even keep up is lady, in the anime, patty sees Dante fighting demons through the curtain, yet, by the animes own logic, he could have killed them in 1 second, because every single shot is an instant kill, the demons were not on the other side of the curtain and Dante knows how to use a gun, so he didn't stop himself, even if it was all sword work, he could have been like, stinger. basic combo to instantly kill ten guys. stinger. repeat. done in 4 seconds.
i think that it was slow to patty (an entire minute or so!) because either, sid was using the powers of another demon to place patty in an bubble of slowed time, or he actually used another demons power to literally hack Dante's and use that to make patty and him move normally while Dante took waaaay to long to kill some random guys.
and nero may have it as well, consider how often things go slow mo for the guy, in the opening sequence he's fighting demons and bang! slow mo! then when Dante uses break ceiling like some lame assasin (which was super effective, by the way) boom! slow mo! nero does not know how to control his demon powers fully, yet it always happens when something dramatic or emotional occurs (kyrie is taken away = despair, demon fight = desperation to arrive before it's to late, about to die or get hit (that happens alot, if i remember correct, shaky on this one) = depseration, panic, survival instincts, or possibly all three.
when Nero went rage with yamato, agnus also devil triggered, cancelling out Neros power with his own (or at least reducing), yet there was a moment of stop motion just before he went and obliterated those bianco angelos randomly(much weaker demons, much less able to resist his charms, i mean powers, yes, i just came up with bianco and nero romance for a cheap joke.).
so in translation, Sparda wold move stupidly fast and oneshot Dante.
two, if he's lucky.
