It seems to me that there are a myriad of things one can do to improve accuracy and speed. And it definitely depends on what you are using that speed to do.
For example, if what you want to do is play a piece memorized at a freakishly clean and fast pace, then practicing at a extremely slow pace with dead even/fluid strokes and smoothness of tone is a great way to go. Once your brain figures out what you are trying to do, you will be able to slowly increase speed until lightning is the only thing faster than your hands.
If, on the other hand, you are looking for the fastest hands when improvising, there are lots of techniques that will help with this. One of my favorites is to switch up directions with my right hand picking. Let me explain. We all use alternate picking technique right? That is to say, we all alternate down-up when picking with the down stroke usually on the beat. Then try (when practicing) to start with a right hand up stroke on the beat, but still alternate strokes. What will happen is that your right hand will learn to become independent of the beat and will have much more control. Do the same thing when practicing cross-picking. When you can pick opposite strokes as fast and easily as conventional down-up strokes, you will see a pretty big increase in speed and accuracy.
For your left hand, you may want to try a technique I developed about 37 years ago called "touch technique". The concept is that folks often press too hard on the strings with their left hand which causes all sorts of problems (muscle cramps, slow movement, exaggerated movement...etc). Touch technique teaches the hands to press only as hard as needed to produce a clean tone and also teaches the left hand to relax while playing. It works like this. Instead of pressing down each note (in scales or single note tunes), just lightly touch the note with your left hand fingers so that the note sounds with a muted "thunk" when picked. If you actually hear a clear tone or fret buzz, you are pressing too hard. Each note should be completely muted. What you will notice right away is that your fingers will NOT jump away from the fingerboard, but instead will stay close to the fingerboard even when they are not playing. If you practice this regularly, you will find that your hands will play faster and much more relaxed.
Those are my two favorite techniques of faster hands while improvising.