Years ago, I had the privilege of studying violin/viola with a very well known and respected performer/pedagogue. I once asked him how to approach practicing the fast passages in my repertoire. His response was, " practice fast. How do you expect to be able to play fast if you don't practice fast?"
There is certainly a great deal of value in practicing slowly and, admittedly, the bulk of my practice is approached through slow deliberate playing. I think, however, that some time must be devoted to playing at very fast speeds in order to develop the reflexes needed to play quickly. Iznaola, in his book, states that, in addition to traditional scale and arpeggio practice, one should push speed levels through practicing speed bursts. First, this should be done with a very limited number of pitches (3-4). Gradually, additional pitches should be added until high levels of speed can be maintained for longer durations. The teacher mentioned above gave me similar advice and I believe that Tennant mentions speed bursts in his book too.
As a final disclaimer, I believe speed development is dependent on multiple factors and there is no magic exercise that will provide instant results. Practicing speed bursts should be used in conjunction with a variety of technical exercises and approaches.