Every single thing that I’ve learned to play fast on the guitar came from a pretty annoying period of playing it sloppy. The area where people get into trouble is thinking they can be somewhat sloppy and tense. Martin Miller talks about this in his popular video on speed; he says if you tense up you’re engraining false technique, and he’s very correct about this.
However, you can’t do a motion perfectly if your body doesn’t even know how to do the movement yet, and periods of experimentation with loose technique is, as far as I know, the only way to get there. Think about it like this: If on the first day of practice you were doing 3NPS alternate picking with the exact motions that Paul Gilbert used, you would be playing as fast as Paul Gilbert on day one. So in a general sense that is why “start painfully slow and speed up” is literally some of the shittiest advice you can give for the technical aspects of guitar playing, because it assumes all motions at all speeds are exactly the same. Sure, faster motions are often just sped up and Troy has talked about this, but pick depth and slight changes in the hand and arm are very visible once speed increases, so it’s a fool’s errand to think there are zero changes in technique at say, 60 BPM 16th notes, versus 200 BPM 16th notes. Even if the physical changes were zero, the mental shift in perspective would suggest practicing at those two speeds are worlds apart.
Basically, what you need is to get a basic gist of the motion. Over time the pattern is something like this:
Experimentation -> Failure -> Failure -> Click -> Failure -> Click -> Click -> Failure -> Click -> Click -> Click -> Desired Performance
The movement clicks at some point, and from there further smoothness gets you insane speed.
Proponents of metronome practice, on the other hand, often miss the forest for the trees and trade smoothness for bumping up that metronome. just. one. more. notch. Not only that, they add another variable to the equation which is human judgment in deciding whether or not to increase the metronome speed. The funny part is the body already does this, and this is a fact in neuroscience: The cerebellum assumes that any sequential input within a timeframe (no research to indicate what this might be) is to be played as fast humanly possible by the person playing it. Every time you perform a repetition the brain is essentially thinking “How I can make this faster?” You can look up Dr. Frank Wilson for more information on this. Really, you need to play at medium and fast speeds that are often uncomfortable and sound not so great. You just cannot get there otherwise by trying to be perfect all the time.