People who know japanese, if you listen to The Disappearance of Hatsune Miku can you actually understand the lyrics without looking them up?

Comprehensible fast singing is a real skill. It usually involves hitting constants much harder than you'd normally do which is unnatural - when you try and speak fast most people will mute their constants. A great example of someone who manages this is Busta Rhymes, even while moving quite quickly his lyrics are extremely comprehensible.