I've seen this work many times.
Try singing the notes faster than you can play; do this without the guitar in your hands. Then try playing them while you sing fast. This technique addresses what is known as the covert motor routine (the governing command to play fast) instead of the covert motor routine (which is the actual commands to move the each finger, ie; the things we can see). Sometimes we simply are not really imagining/feeling the music fast enough.
Dr. Lawrence A. McDonald, D.M.A., Art Kaplan Fellow
Author of The Conservatory Tutor for Guitar
2008 Michael Thames Cd/Br
Royal Conservatory Advanced Guitar Instructor
Royal Conservatory Advanced Theory Instructor