I took a few lessons from Steve Stine, and the exercise that he gave me was so simple that I felt dumb for not thinking of it.
Simply start doing pull offs. Start with your index finger, and go for 20 seconds. Then fret your index finger and start pulling off with your middle finger for 20 seconds, until you get to your pinky. (you’ll realize, if you haven’t already, that your pinky hates you. ) Then, fret your middle finger, and do pull offs with your ring finger for 20 seconds, then pinky, etc.
See the pattern? By the time you’re done, your forearm will be on fire, and your fingers will be spastic. It’s a great endurance exercise, and it is very useful for increasing speed.