I used to play a woodwind instrument and when I had a fast passage I was always told to practice it slowly first and then get progressively quicker. But usually I could never get to the speed I was supposed to play at, until I discovered another technique:
Just practise e.g. the first three notes at full speed, and at every other repetition and one or two notes more, but stay at full speed from the beginning.
This technique is quite tiring/not very exciting but brought me quite a bit faster to my goal.