Relative speed, folks.
"But no matter how fast your brain can process something or your ears can hear something, when you're speaking at those speeds there's nothing you can do about making your voice/words not sound like that lol"
That's not how it works. If someone is coming toward me at a speed faster than the speed of sound, say Mach 2, their words are still going to come toward me at the speed of sound. Meaning the sound of that person's voice would arrive after they reach me. The sound of their words isn't going to be moving any faster than the speed of sound.
Now let's say that shinobi somehow learn how to speak faster than they normally would to mitigate this (which is impossible, but whatever). Because their brains are plastic, their auditory cortices would acclimate to processing speech that occurs at a higher rate than what they were normally used to.
It's the same reason why if you hear a song where someone is speed rapping, you won't be able to understand them at first. But if you repeatedly listen to it, and even read the lyrics, it becomes easier to understand it over time despite the rate at which the words are spoken not changing at all.
It's related to why someone speaking a different language sounds like their speaking very quickly. In that case, the average information density per syllable and the number of syllables spoken per second is different than what you're used to, so your brain doesn't know how to break the syllables into meaningful fragments and it comes out as gibberish.
But that changes once you learn the language.