I’d asked @Troy about using his skills to crack this. He was interested, but I know he’s super busy.
I’m a semi retired amateur classical guitarist that studied formally at college. I’ve put tons of time into this and all I can guess, just knowing what we know about motion speed in general, is that if we’re not getting close to Paco De Lucia speeds, we’re simply doing it wrong.
Some day I plan to circle back to this. I’m currently having too much fun with with plectrum electric and steel string acoustic to take such a huge detour (plus I love not having to keep up with nail care). When I do get back to it, the approach I’ll take is to just start with as much footage as I can find of the very best. I’m not hugely immersed in that genre, but from what I’ve seen there are none better than Paco De Lucia and Grisha Goryachev. I’ll be paying attention to their hand position, curvature (or lack thereof) of their fingers, the range of motion they use, the path of the motion they make (e.g. are they making super squashed semi-circles, or is there some “trick”, like as one finger begins pressing the string down, prior to going through it, this allows the finger that just played to make a straight line returning to its prepared position, because the string is not in the way). Stuff like that.
If it’s about just “learning the motion”, it’s possible bursts help figure that. I’d ditch the metronome though. I think that will be giving you “some other thing to worry about”. You can always dedicate time to rhythmic accuracy after learning the motion.
I’d consider this dogma and ignore it. What’s most likely happening is the same thing that happens when Rusty Cooley hits his high gear - the motions are getting smaller just because they have too. The small motions aren’t creating the speed, they are a byproduct of the speed.
To me, this is problem #1. If a raw motion can’t go as fast as the one you’re trying to make, it’s not the right motion. I don’t think that drumming the fingers on the table is the same motion as picado. More realistic is allowing your fingers to dangle over the edge of a table and tap on the side part of the table top.
Something like this:
That’s more realistic as to how you’d be hitting the strings. Also, notice how I’m doing just index over and over, also just middle over and over. They can go pretty fast on their own so it may be a matter of being aware of that, then just getting things synced properly regarding how one finger releases as the other prepares.
Who knows if anything I’ve just said has any merit, but if what you’re currently doing doesn’t work, at least this is some other thing to try. I didn’t clock it but just trying something similar to a metronome, it’s above 180 bpm so something about that motion is “good”.
Anyway, good luck!