I made up some exercises that helped increase my left hand speed:
Finger the following chords in order without strumming: G, D, E, A, C, repeat. The chord shapes are formed and positioned in such ways that it not only warms your hand up before practice, but helps your muscles get used to forming shapes quicker. The reason I say not to strum is because then you'd be worrying (consciously or subconsciously) too much about how clean the hold is rather than the position. Once you feel comfortable with your speed, then start strumming each chord because speed is nothing without proficiency.
Practice hammer-ons and pull offs between all your fingers. Start slow with frets directly next to eachother, then pick up speed and stretch them out while maintaining the same force until you reach your limit. This will strengthen your fingers, practice the techniques, and help your speed.
Play a song you already know but faster, then play it faster, then faster, etc. Make sure you can play it as perfectly as possible each time before you speed it up again. This particularly helped me with speed, and also helped me learn to songs faster because I knew how to play more quickly. Being used to fast paced playing has helped me decipher picking patterns among other things more easily.
The final and most important exercise: Practice until it hurts. Play every day even if your fingers feel like they're being cut off, even if it's just for a few minutes. Once you develope those callouses... Practice even more. Your body developes muscle memory, and playing constantly is basically like doing brain exercises for your hands.
