Should I practice Fast or Slow for Technical issues??

[bronnestam]

@ falala & nick

At the risk of sounding like a nerd, very often, especially in the virtuoso repertoire, the best solution to a technical problem is SKIPPING notes (filling them in much later), provided that the basic rhythm of the piece and the body dynamics of motion are maintained.
P.S.: You thus create long lines in your head and your body starts feeling where it has to go. The skipped notes then easily fall into place. Sometimes this solves a problem in 10 minutes, while the traditional training of all the correct details at once could take months.

I also learned "skeleton practice" last fall and I found that it decreased my learning time to a fraction of what it used to be.
You can learn simplified versions of difficult pieces that are over your present level ... which is fun and rewarding. No, it is not blasphemy. The only thing that is important to remember is that YOU MUST KEEP THE ORIGINAL FINGERING if you want to learn the whole, complex version later.
From a pure psychological POV this is the most logical way to learn. If you want to write a novel and have not done it before, it is most unwise to start with the first paragraph and polish it to perfection before you write the next one ... and the next one ... Instead you make a rough draft, perhaps you start with some interesting scenes you have had in mind, or you write a synopsis, and if you meet an obstacle you write like this in your manuscript: "Tom goes to see Betty [add more about her house here, perhaps?] and then Daniel unexpectedly also shows up [why did he return earlier from Afghanistan? Find a good reason! Also check out the policies for homecomers in the army.]"

In this way you will not get lost in the jungle of details while you work with the major plot. Which, in turn, also means that you can work calmly with your details about Betty's house later on, when you already know that your story has a firm structure which works well, and when you know exactly where you're going. Perhaps you can find details when you describe her garden that alludes to the catastrophy which will come at the end? (Now, this is the funniest part in storytelling!)

A way to learn a complicated arpeggio could be to rewrite it as chords, perhaps simplify the chords a bit, and then you can play them at full speed almost at once, hence getting a good feeling of how you should move your hands. Once you've learned to do this, you can go back to work with the details in slow tempo and with confidence, because you know exactly how to connect the beginning and the end of the difficult section with the rest.
It is also advisable not to practice the same "skeleton" too many times, but to vary it a bit after a few attempts. You don't want to get stuck in your draft.

Another disadvantage with slow practice is that it is booooring.  Probably you won't mind doing the refining and polishing and the tedious little technical help exercises when you are already "half-way home", but in the early stages of learning it can be a killer. First, you must get a grip of the whole piece - right tempo, right expression, but simplified notes. 

But once you've got there, and you have found some technical issues - then slow practice is essential, IMO. Then you will not find it boring. You must also regularly practice pieces you know well in a slow tempo, in order not to get sloppy over time.