Playing fast well requires a gradual ramp up in speed in manageable intervals from slow and steady, to fast and steady. This is what I've done for a few flamenco guitar techniques like rasgueo and picado runs, inspired by well-worn teaching methodologies. Pick up the book The Musician's Way, or read the Musician's Way blog if you get a chance.
How do you ramp up the speed without sacrificing quality of sound, tone, attack? Here's a little trick to do that when you work technique into your practice schedule.
Let's assume you've isolated a hurdle you are trying to work past, which to you is a problem related to speed, but could in fact be masking an underlying technical problem that needs to be worked into your muscles, strengthening them at a slower speed.
Find how you want to fit the speedy riff that was difficult into the metronome's time: 3/4, 4/4, etc.
Start at a comfortable speed on the metronome, probably somewhere under 100 bpm
Play the riff at least 5-10 times on the <100 bpm speed, listening to any inaccuracy of execution.
Speed up 10 bpm, play same thing 5-10 times, speed up 10 bpm, play the same, speed up 10 bpm play the same.
Take a break and put it down, or come back to it later. This should envigorate your practice, not wear it out.
Write, post, publish, prophet!