When I first returned to memory work, I started researching memorization techniques on the internet. If you've found this blog through a similar search, you know that there's quite a bit of information out there. (I will say, however, that one of the reasons I'm writing this blog is because most of the information out there is cursory--and, as you know by now, memorization practice is not a cursory endeavor.)
Back then, I came upon a site (reference forgotten) that briefly discussed how to work on slow pieces. The general suggestion was to play them quickly. The logic, as I understood, was to establish tactile memory as a foundation and then eventually slow the piece down to the desired tempo.
You already know my opinion on establishing tactile memory as a foundation! But let's explore what else is wrong with this approach, not only because it applies to slow pieces, but it applies to all repertoire as well.
It is true that learning a slow movement by playing it fast can work. I've done it, although I no longer do it. But what is it we are trying to achieve?
By learning and playing slow pieces quickly, we are affecting the manner in which we will ultimately perform them. Just as listening to recordings will influence our personal interpretations, this method will certainly get in the way of our ultimate goal of transcendent performance. In short, it's unmusical.
Aside from this highfalutin talk, there is the practical issue of the very nature of slow music: Slow music is slow, making it easy to lose one's concentration and get lost in the lack of activity. Therefore, slow music requires the highest level of visual and aural memory. Building a tactile foundation in order to achieve this will ultimately fail.
Personally, I use a slow metronome to work my slow movements. That metronome is set at a speed slower than my ultimate tempo. I do this for my mental work and also my work at the instrument. The metronome keeps me from speeding through the long, slow notes--both mentally and physically.
You know what? I do the same thing for my faster movements. All in all, I apply the same approach to my repertoire, regardless of final tempo.