Seems a bit wanky if you ask me. If his speed was immeasurable I don't think he'd be having all this trouble with Sans, or characters that would otherwise be considered weak to universal. Killing him past death would be pointless, he'd come back either way, and if we are going with ALL of his abilities, Kumagawa could land a book maker screw purely based off of imagination. That's the difference here, Frisk can't put down Kumagawa for good, and Kumagawa can't either. But by breaking his mental state, and lowering his will Frisk will not be able to use said Determination. If I'm not mistaken though, I'm more than sure his actual speed and reaction speed are a lot different. I don't recall any moment where Frisk was going FTL, and running Sonic style through the Underground.
Also, how is this even counted as a loss, I'm pretty sure that this is an inconclusive battle here. Frisk isn't putting Kumagawa down for good any time soon, and Kumagawa is unable to kill Frisk for good via erasing him.