180 16ths and 120 sextuplets I think is a good starting point for shred for two reasons:
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I think if you hear performances isolated with nothing giving the tempo, like the click track or drums are muted, I think it would still sound plenty “shreddy.”
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Technique wise, 180 16th notes, as an educator, is my personal benchmark for if I have a student that can hit 180 with a tremolo and it feels easy, that means they probably have an efficient picking motion.
I’d also throw out there, similar to @tommo 's point, theres plenty of playing below these speeds that, depending on context, still sound “fast” to most listeners, especially if they’re not guitar nerds. A lead like the Sails of Charon solo kinda falls into this category for me. Its only sextuplets at 110, but that first run definitely “hits” like a shred lick over that groove.
I do tend to have a distinction between “advanced metal lead” and “shredders.” I don’t know if that makes sense, but a good example would be like, Randy Rhoads was a very skilled metal lead guitarist, but Yngwie is a shredder. Btw, I love Randy’s playing, so thats no slight at all, I’m just referencing speed and the lines that are being played.