Back In Black isn't a song. It's a divine call that gets channeled through five righteous dudes every thousand years or so. That's why dragons and sea monsters don't exist anymore.
View post (What do you classify as fast?)
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I pretty much agree that anything 150+ is fast. Improvisation in terms of permutating every single note becomes extremely difficult at around 180. Keep in mind, however, that after that speed most of the time you aren't really going for any sort of melody, you're going for a texture that can fill the soundscape well--almost like an impressionistic pianist would imply a certain key or tone with different textures.