Mr. Neunzlingr doesn't actually play it very fast. It could conceivably be played faster, but what he does is play it at what might be a minimum tempo appropriate for the 6/8 time signature, i.e., so that the events of each measure can emerge as receivable by the ear as organized into two macro-pulses per measure, each consisting of three components. More concisely, that's the principle that 6 eighth notes are counted but there are actually two beats per measure, translated into wordinessese. Were it any slower,each measure would tend to sound more like six isolated events rather than an overall melodic gesture, though the patterned raise and fall by which it is composed would mitigate against the structure dissipating away entirely.
A tempo which achieves that articulation of the structure is your aim, but that being said, if you're not ready yet to do so, then what you want to do is not to continue unsuccessfully to match it, but neither is it to tie yourself to some dictatorial external reference like a metronome. Instead, what you want to do is find your own subjective "cruise tempo". This would not be the fastest speed at which you are able to play it while scrambling to keep up with yourself, but the maximum speed at which you are still able comfortably to control everything that is going on. You needn't eschew a metronome entirely, but use it as a check reference at intervals of several days, or a week, just to see how your "cruise speed" has developed after having "lived there" for a while.
Meanwhile, there are some other issues with this etude and with Sagreras in general to consider. The Sagreras method is a terrific progression of studies, but there are some caveats to it. One is that it takes considerable probing to discover what each etude is "about", or, what it can be about beyond the apparent surface of a prosaic exercise. The text that accompanies the etudes is pretty much useless for such exploration, which can be an examination that reveals that there is scarcely a one of the etudes that does not benefit from modification of the little that Sagreras does say, or what the notes themselves imply. Perhaps he deliberately held back from abundant explication, feeling that such should is actually the province of an assumed actual instruction sessions with the student. If so, this leaves inexperienced people working on their own with this method somewhat in the dark much of the time, or under the false impression that once a mechanical reproduction of a literal reading of the score is achieved, then he is done with extracting everything the etude can offer as an exercise.
This does not man that it is a futile enterprise to teach ones self from the method, but it is more problematic. Just as suggestions, here a some questions for you to think about: I notice that Mr. Neunzling does not follow Sagreras' indicated right hand fingering exactly. Specifically, I am referring to what happens in the indicated fingering in moving from measure four to measure five. There is a break in the printed fingering pattern there-- what do you think that is about? Is it a misprint, or do you think there is a fingering principle at work here? it is deliberate on Sagreas' part, then compare this etude with exercise 36. If you were to discover and articulate any sort of guiding principle for the spot I bring to your attention in exercise 42, then is that principle transgressed in 36? If so, why? Are there different considerations at work?
And, what of Mr. Neunzling's having disregarded the fingering? Was he just careless, and didn't notice? Or is he adhering to some other principle? Is he "wrong" to have done so?
Musically, what is going on this etude as a composition? Does it, in fact, merit the elevated connotation suggested of the term etude, or is it nothing more than a "mere" exercise? If it is more than a mere exercise, then how does each measure fit into and relate to the arc of the etude as composition? What means does a guitarist have, or does the instrument provide, that can be utilized to express the construction of this etude as a "piece" of music? Primarily one or two, or several? If in number, can they be isolated, and then recombined to work together? Did Mr. Neunzling do everything he could have done in order to realize the potential of this piece?
Finally, do you think that all I am suggesting here is overwrought, overthinking that is too much to bring to bear on a simple little etude like this, and that interpretive matters should be left until later, when one is working on repertoire more "deserving" of such approach? Or, is it possible that interpretation, just like technique, can be presented in basic incipient form, from the very beginning of learning, and that there are "techniques" of interpretation to be developed that are just as legitimately pat of what an etude is 'about" as are he mechanics involved.
I suppose I've pretty much tipped my hand in terms of what I think about such matters-- more imprtantly, what do you think?