After years of speculation, it finally happened. Tech N9ne ft. Krizz Kaliko & Eminem - Speedom (Worldwide Choppers 2):
EDIT: Being able to rap fast is easy. Especially when it comes to Twista, his raps are usually just on the beat, beat, beat. Count, count, count. In other words, very damn predictable and boring. The real talent is doing everything we know and THEN rhyming off beats, etc. etc. There's no way to explain this in words, except to rhyme in real life.
With crazy underground rappers, I CANNOT replicate their rhymes just by reading their poetry. I have to listen to them. See where this is going? Hip-hop is a dynamic art. Period.