While Matteo makes a very good point, I do understand the draw of improving your speed. While you are learning guitar, being able to play faster gives you "instant gratification", and boosts your confidence.

For me, I'd probably learn a scale to where I can play 16th notes at around 120bpm or so, then move on to another scale, HOWEVER, during my daily/weekly practice routine, I'd spend 5 minutes or so working on my speed of that scale (with a metronome, of course), and then move on transitions, working on new scales, etc. etc.

It's not such a bad idea to plan out your practice routine (as long as you are always changing it up from time to time).

Anyway, that's my two cents...not that I am some tremendous speed-player...but GT has some great speed building tutorials!

Happy playing! :)