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Just because Dante is now stronger then Vergil was doesn't mean a thing in terms of speed. Dante has fast reflexes as evident in DMC3 with the billiard ball shooting and DMC4 with the lightning demon. Dante has never been as fast as his brother. He's become stronger but not faster.
It kind of does for Dante in DMC4 has displayed speed feats far more impressive than what he had in DMC3 meaning he became faster and the fact that he could still beat Vergil in DMC3 is evident that the speed gap between them in DMC3 wasn't that far.
Plus when Dante unlocked his DT in DMC3 one of the first things he could now do is run much more faster meaning stats such as speed, stamina, durability, and strength isn't tied to a certain individual. They're both the sons of sparda therefor making them equal. Vergil in DMC3 wasn't just faster he was stronger, more durable, and overall better until the near end of DMC3 where they became equals. I mean if Vergil so much faster he wouldn't be getting slashed up and Dante wouldn't be able to keep up so easily in their second fight and win in their third fight meaning Dante in DMC3 was either just as fast or almost as fast.
So Dante can become faster if he wanted to and he did. Plus reflexives is just another form of speed. Perhaps in a race from start to finish Vergil would finish but in a pure combat sense Dante in DMC4 has lightning quick reflexives as well as lightning quick movement that he can literally disappear and reappear out of nowhere and blitz opponents like their snails.
Even though Dante was holding back his feats are as still followed. Dante by OBD terms wasn't "Bloodlusted". Bloodlusted means when a fight is set where both contestants or fighters are out to kill the other and taking the fight seriously. By OBD rules Bloodlusted doesn't affect their aforemonted feats. Dante was out to stop and subdue Nero and hold back fearing he could kill him we don't know if he was fighting much slower than usual. Plus he was panting meaning Nero managed to give him a work up and that was when Nero attacking at pure rage where Nero fights much better when he is cool headed.
Regardless Nero is no slowpoke considering he easily defeated Credo who made sonic booms when he attacked and moved and could beat Blitz meaning he is pretty fast himself not as fast as the Sparda Bros or can handle speedy opponents.
Now the real argument is if DmC Vergil is as fast the old Vergil. I say no because lets put it like this. Both we're defeated by their brothers but Dante in DMC3 actually got better and faster when he unlocked his Devil Trigger (shown running at such speeds down Temen-ni-Gru's tower) meaning Dante in DMC3 had to prove such amazing speed feats in cutscenes and he even gained the ability to WILLINGLY stop time for a few seconds so whatever speed Vergil is moving at he can stop him in his tracks so Dante had feats that rivaled Vergil's hyped up speed. Whereas DmC Dante just beat Vergil without showing much of a speed improvement or showed how fast he could be. Although during his gameplay in Downfall he showed he could be speedy in battle, he does have greater attacking speed, but movement speed is on par with that of DmC Dante's well a bit faster and even the gameplay of DMC3/4 seems to be at the same speed as the gameplay of both Dante and Vergil in DmC but we all know DMC4 Dante is far more faster (or the fact you're slow as molasses in DMC2....well an exaggeration but story wise DMC2 Dante is the fastest as he displayed amazing speed feats via cutscenes that aren't possible in gameplay) so you can't fully use gameplay to assess one's full stats because gameplay is dependent on the player or limited design aspects and in the case of a OBD match that factor isn't included and gameplay normally contradicts narrative if you look closely into it (like Cloud could've used phoenix downs to save Aerith or Dante or Nero could escape all their problems with a good Holy Water or Gold Orb). There is no hints or clues that DmC Dante or Vergil could access Hypersonic (which is mach 10+) speeds for there was no display of it in cutscenes and short term gameplay to make such an assumption. I mean I haven't see Dante run so fast that he broke the sound barrier or Vergil pull off a series of Judgement Cuts all over the place while moving at blinding speed.
OBD fights are specified and debated based mostly on feats displayed in cutscenes, narrative implications, and a general gameplay (meaning what types of attacks and moves they can pull off that aren't shown during cinematics and to DMC's credit they usually display the effects of Dante and another other character's new weapon, upgrade, and even abilities in cinematics to display them and how would look and work in a cinematic like Dante doing Twosome Time and Rainstorm when he awakened his DT or stopping time with Quicksilver in DMC3 although to DmC's credit gameplay is easily tied to cinematics). As well as the fact there are times when cinematics display greater feats that the player can't do.
Its pretty ironic that with DMC the characters seem more OP in cinematics than in gameplay while in DmC the characters seem more OP in gameplay than cinematics.