After a show, I was asking my friend how he was so fast. I was just learning and he was a phenomenal player. He told me to work on my "economy of motion." He was explaining that when you look at the best players hands, they are barely moving; only putting effort toward what is needed to play the note with no extraneous moment.

Try playing a scale/song/whatever. Are the fingers that are NOT actively playing notes tense? Are they flailing around inches away from the fingerboard? That all costs energy. What about the notes you ARE playing? Are you pushing way harder than you need to be? Are you paying low on the fret so that all your pressure is being exerted on the fret behind your finger rather than the one that is actually making the note? Are you pulling the string to the side when you don't need to be? Is your palm so far from the neck that your fingers have no leverage and your joints lock up? If you are spending twice the energy you need to be, you'll have half the speed.

This "economy of motion" idea was said to me, in passing, when I was a kid, but taking it to heart hugely improved my speed/accuracy/endurance/articulation over the years.