Rather than playing slowly just to play slowly, I think it's wise to consider what the actual aim of slow playing is. The aim of playing slowly is not simply to enable one to play quickly. I think that the actual physical aim of both fast
and slow playing is found within the same, fundamental, kinesthetic experience; "complete" comfort and ease. Any kind of tightness is the arch-enemy of speed playing.
There is nothing more magical about scales and hanon than there is about a musical passage, in terms of making our mind and body understand what is required of it to be comfortable while playing. The "selling point" behind spending hours, days, weeks, months, years, and even decades in the "study" of these exercises is that it is supposed to help us in our repertoire and overall skill. If that is true, then there must be something transferrable from exercises to repertoire, meaning, there is the same fundamental, physical value within both. If that were not the case, then there is even less reason to spend time in mere exercises like Hanon since there would be nothing transferrable from it to repertoire. If it IS the case, then why not go straight to the music since it contains the same physical principles and is what one is truly after anyway ? I guess that is a bit of a side-note.
What one is achieving in slow play must, by necessity, have transferrable data into fast playing -- otherwise there is no point. The only physical reason to play slowly is to develop comfort and ease with each finger, hand, body position and motion -- when this is achieved, theoretically, one has the dominion to play in any manner (including speed) that is required by the music (assuming that the composer knew what s/he was doing when writing).
If an individual has truly achieved this comfort and ease, there is no reason to speed up in several increments, because it is as simple as binary code -- you are either in/on or out/off of comfort and ease -- whatever the tempo. It is better to grasp the comfort at slow tempos (if necessary), and then play 'at tempo' in small sections (eventually combining the sections), always aiming at a very similar kinesthetic experience as the slow playing produced. You will feel it if you are doing it correctly, and you can trust it, too. If we do not develop this comfort in our slow playing, we will certainly not have this in our fast playing, and the faster we play, the more frantic the experience will be.
Once you have discovered a kinesthetically pleasing way to play a passage, it is your's forever and you have something to always compare what you are doing to. It must become the very essence of your work, and everything else in terms of physicality is concerned, must be formed and molded in order to freely allow this kinesthetic experience. Finding that particular comfort is not something anybody can *make* happen for you -- not even hanon

.
Try sitting at the piano, perfectly balanced in your body and on the bench, with both your hands fully resting on the keys. You should not feel a gravitional/backward pull that makes your hands want to slide off the keys -- if you feel this then you are sitting too far away, and note, if we play like this we are fighting gravity and ourselves, constantly causing our body to compensate for this (and this is "static" in terms of developing a clear, kinesthetic sense of comfort).
You should be able to completely rest your arms, shoulders, back, butt ... everything -- and, if you do this correctly, you should experience a sense of "oneness" with the instrument. That "oneness" is present only when there is not any kind of "static" mixing into our kinesthetic signals and experience. I personally first found this sense of kinesthetic comfort in this way (though at the time I didn't know what it was nor what it meant), and at times in the past, if I have gotten really mixed up, I take a moment to center myself this way (but it also is a WAY COOL feeling, so sometimes if I am scattered in my life I just sit at the piano like that for as long as I want).
In essence, we should be able to feel this same sense of comfort at the instrument -- no matter what we are playing, and no matter how quickly we are playing it. If we are not, then we need to re-evaluate our approach. I am personally still in the process of perfecting this, but the more I delve into it, the better I feel, and the better I sound when I play (it seems

), whether it be fast
or slow.
Hope that helps

.