i think the main problem with people that practitce but can't seem to play any faster is tenseness, whether in the fingers, wrist, shoulder, foot, whatever. Some players just try so hard that they forget to relax. One guitar teacher i found (i wish i could remember his website) called the feeling of playing fast "the incredible lightness." this is literally what you have to feel. so, next time you are playing and are not able to achieve a certain speed, stop yourself in the middle of a run and check to see if any part of your body is tense. If it is, take a second to relax, and try to play the run like you were about to go to sleep (it's an exagerration, but you what i mean)
I once played with a guy onstage that would lift his foot off the ground like a dog taking a piss every time he hit a high note with his voice and was playing guitar at the same time....
That said, keep practicing and keep on rockin'! peace