Supposedly Mirio Togata was able to defeat half of the Class 1A students in less than 6 seconds. How is this possible? His quirk is supposed to be permeation, does he for some reason also have superhuman speed as part of his superhero habilities?
There are two reasons why Mirio won with that level of ease against Class 1-A.
First, he trained his quirk extensively, and has extremely fine control over it. He avoided all long range quirk attacks by letting them pass through him, but solidified at the right moments to deliver powerful punches.
But this alone wouldn't explain how he managed to do that in 6 seconds. The real reason for his "speed" is a side effect of his Permeation quirk, which is explained at the end of the fight.
His quirk allows him to pass through solid objects, but if he releases the quirk while there is overlapped with another solid object, he is randomly catapulted out. This instantaneous repulsion effect is what he takes advantage of to mimic 'teleportation'.
It is not an additional or hybrid quirk, and he still has to angle his body correctly in order to appear where he wants. As Ashido rightly puts it "It's like a video game glitch".
Sources: Chapter 124 of the Manga, and Episode 25, Season 3 of the anime.
As raised in the comments, this doesn't fully explain how he was able to 1 hit KO Kirishima, who has hardening, with just normal strength.
While unclear exactly why he is so strong, the wikia does list "Enhanced strength" as one of his abilities. Tamaki mentions that he's been strong since childhood, but that he doesn't know to hold back. Nejire also says his "permeation form" has gotten stronger. And Mirio himself mentions he has trained to counter his weakness (people attempting to attack him when in close combat).
It is also possible he was too fast for Kirishima to react in time. Even assuming his catapulting adds momentum to the punches, it seems unlikely that he'd be able to 1-hit a hardened Kirishima.