I have the answer! It's guaranteed to work and you'll sound amazing!
*****This exercise is geared toward students who can already alternate i & m through scales easily and comfortable (not necessarily quickly) using rest-strokes. If you cannot do this yet, please READ the exercise but DO NOT TRY TO DO IT! (The idea behind this method can be used for all techniques so it's a good idea to at least understand the concept.)******
Speed Bursts (or Note-Groupings)
I. FOUNDATION FIRST (VERY IMPORTANT )
Before you can begin to build speed with scales on the guitar, you need to first be able to play alternate fingers (i & m) on open strings correctly, precisely and with good tone and dynamics.
A. Play 10 notes on your high E string (1st string) by alternating fingers "i" and "m" with rest strokes and bring the speed up to a comfortable, medium, tempo.
B. Then start with your "m" finger.
C. Once you can you do this, with good tone and proper technique, you're ready to start working speed building drills. If you can't do this yet, you can't work on speed building. (Pleas see your teacher)
II. 2-NOTE SPEED BURSTS (OR NOTE-GROUPING)
A. Play 8 slow and even notes, ALTERNATING i & m, on the first string (good tone, good technique, stay relaxed)
B. Rest for 2-5 seconds.
C. Play 2 notes as fast as you can so that you hear barely hear the separation between the notes. You want to think of the 2-note burst as one movement and almost as if it were 1 note. Think of this as playing a grace note and primary (target) note as close together as possible. Percussionists call this a "flam." Here's video on what it sounds like: https://www.youtube.com/watch?v=fP6FFjZckMs.
D. Rest for 2-5 seconds.
E. Repeat on same string 4 times MAKING SURE TO REST FOR 2 SECONDS between each speed-burst repetition. During the 2 second rest you need to relax your hand & body and quickly analyze what you could do to improve the next speed burt (e.g., "I need to articulate my m finger more).
E. Repeat on the next string starting with 8 slow notes and then applying your speed bursts. Remember to rest for for 2-5 seconds between each speed burst.
G. Follow this method on all strings (1st string to 6th and then 6th to 1st) once starting with "i" and then starting with "m"
IV. I'll write a "Part 2" to talk about how to apply this to scales. Stay tuned!