Hess states, “Most guitar players slow themselves down while picking without even knowing it. Fortunately, becoming a fast guitarist is easier than what most people think. To play with an extremely fast picking technique, you just have to follow this very basic concept: 1. When you ascend strings (move from the lower string to the higher string), ALWAYS use a downstroke – regardless of what notes come before or after. 2. Anytime you descend strings (move from the higher string to the lower string), ALWAYS play with an upstroke. This is called “directional picking”.
Hess then goes on to state several reasons why most guitar players do not make the necessary changes to incorporate the approach from above:
“Guitarists constantly make excuses for not using directional picking in their playing, and their overall skills remain limited for it. Don’t let yourself make the same mistake. If you ever hear yourself using the following rationalizations, you know you need to change: 1. I have to master alternate picking first before I can move on to directional. 2. My favorite guitarist uses alternate picking with success. 3. It’s hard to string skip using directional picking. 4. Directional picking is for shredders only, it doesn’t suit my style.”
Hess considers directional picking to be the most overall efficient form of picking and recommends it for improving other techniques as well (outside of pure speed picking).
More information on Hess’ guitar speed (http://tomhess.net/