Spoiler
Show
Super Saiyans don't have a minimum ability level--It's canonically a X50 Multiplier, rather than a set "You're base power stops here, then you have to become a super saiyan" thing.
The "They're moving so fast I can't see them" isn't a trait of a specific power level, so much as a trait of one person being massively more powerful than the other--Mr. Satan, whos power level is only 140, can run so fast that normal humans think he teleported.
Krillin had a mini-arc in which he overcame his fears and self-doubt, reclaimed his fighting spirit, and it's implied got a major power boost. He's also apparently been training all to hell and back ever since. The power gap is still there, but it's proportionately much smaller, the gap between Goku and Krillin, than it's been in a long time(Being able to hold his own, even push back, a SSJB level Super Kamehameha with just a normal Kamehameha for a time is a good demonstration. Goku is a hell of a lot more powerful, but Krillin's closer to Goku's full power than he's been in a good long while.)
Besides, it's pretty clear that Krillin was, like Piccolo against Frost, doing so well becuase he was using clever strategies.