Its like you asking someone how to say hello in Spanish. The simple “Hola” is short, 2 syllables, and if English is your native language or something close to the romance languages, its not hard to comprehend. But now if you asked how to say “Hello how are you” (Hola, como esta usted?), that’s where it becomes a little difficult. The threshold for new sounds are low so when hearing a sentence in L2 that’s longer than what you’re used to hearing, a lot of times you’ll hear the response “they speak too fast”. No, that’s normal for them. YOU just haven’t increased your threshold for sounds in that language.

I believe this because that’s what I experienced. Im learning Spanish without textbooks and classes. Just immersion. And of course in the beginning I couldn’t understand anything because I thought they spoke too fast. But Through constant immersion and REPETITION mixed with exposure to new audio, I can comprehend about 70-80% of a news broadcast or telenovela (Spanish soap opera). And even in Spanish, you might have a few dialects in which the seem to speak REALLY fast (example, Caribbean Spanish compared to Spanish in mexico. Very fast and broken.) but the thing is those people are used to it.

Just like Khatzu said, you have to get used to it and in doing that you increase the threshold for the new language sounds.

“I have great vision, but yeah, my eyes tire”
I have this problem too. I’m curious, does anyone here find their eyes tire more when reading kanji or hanzi? I do.