Generally speaking, to play fast, you need to reduce the amount of time between each attack. Doing this by speeding up slow playing does not work. You usually hit a speed wall and will need to change the technique, similar to the difference between walking and running (speeding up a walk does not result in your fastest movement; you have to change to the running technique for that). Instead, you need to find the "running technique" that you will be using for fast passages. This is different for different movements, of course. Here are some models:
1. simple arpeggios are based on closing the whole hand into a fist; this is the model for pami, pima, pim, pmi, etc.
2. complicated arps are based on multiples of #1. pimami could be thought of as pima+mi
3. two finger scales are athletically challenging, and can be approached by flams (playing i as a timeless ornament to m, for example), and then building into 3,4, 5, and more bursts.
4. what about fast left hand position changes? Generally, if you move the elbow first, the hand can be "flung" to its new position almost instantly.
Once you acquire these new techniques, it's generally better to slow them down (but keeping the fast, "running" movement) and practice for accuracy than try to play fast all the time.