Superman is faster than Knull. There is no point in denying that.
There are cosmic entities with zero to none speed feats, but there the speed argument still holds no value – because these certain entities are beyond simplistic concepts such as speed. And I don't think that Knull is one of those entities.
With that being said: speed isn't everything. There are other stats, which matter as well.
Take Scarlet Witch at the height of her powers. She was hurt by an arrow. So she had all this power, but she was a glass canon. There are such cosmic entities. And I don't think that Knull is one of those entities either.
No matter if Sentry was overconfident or not, it takes a certain physical power level to break yourself free of Sentrys grip and rip him apart. There is no way around that. Knulls physical stats must be insane for him to be able to pull that off.
And I don't think that regular old Superman (looking past the modern metafictional way of writing the character) is capable of competing with a cosmic entity on that level. In the end of the day I don't see that regular old Superman beating Celestials either, the way Sentry and Knull have done it. And when you consider what Knull did to Celestials and what he did to Sentry, it kinda becomes obvious how powerful that character is. And how the only thing defeating him will end up being a plot device. At least in a Marvel comic. But yeah, in a DC comic Supermetaman would have been the one to beat Knull.