The best way to improve is practice, practice, practice!!! A bit each day goes a long way and is much better than say, playing for a few hours straight one day a week. If there is a certain piece you want to play, start it slow and you can build the speed up as you become more confident and familiar with it.
Oh, you could also try a grip master - they increase finger speed and agility. But don't overuse it!!! I was a bit gung ho when I got mine and ended up taking anti-inflammatory pills because my hands and forearm hurt like hell afterwards
What Metallica song are you trying to play? Do you play Kirk's or James' part? (ie. Rhythm or finger picking/classical).
Teachers are great, but they can only do so much. If you want to learn theory and how to read musical notation, a teacher is defiently the way to go. However, for a topic like speed and agility, they can only provide advice.
The main theing is can recommend for solo passages is to use pick with your wrist, rather than your whole arm. By using fast, yet slight movements of your wrist, you will not only be faster, but you will not be as tired.
For chords, the best way to increase speed is practice (just like anything else on the guitar). However, by looking for common notes it the chords, you can reduce the amount of movement in switching the chord. If you have two chords and both contain an A. Place one finger on the A, but when switching, keep the finger on the A and only move the other fingers. This will require time to come up with which patterns work best, but once you do it will be worth it.
Do what I did when I first started. Practice for 4 hours each day on just scales, I know it sounds like over doing it, but I know my scales now dont I. Pick a chord and just pick the notes nice and clean and slowly increase your speed.
As for how do I relax when I play, to be very very honest about this, I just dont give a !***! what people think. Thats the best advice I can give you.
Get your scales down, it helps. Just practice whenever you can, practice the 5 positions of the penatonic. You just get faster and faster through practice, just start trying to get the cleanest sound possible. Go as fast as you can without sacrificing clarity, then slowly push your limits. GOing fast and sloppy just makes you sound bad.
Thanks for the advice everyone, i am going to go buy a gripmaster, play scales for hours and and play slow. The wrist idea sounds pretty siple, i dunno why i didnt think about it myself!! In answer to the first reply, i try and play it all, i suck at rhtyhm and solos at the mo but prefer rhthym as i dont think i will be able to do solos quick enough. Cheers peeps.
Scales are how you derive the chord in the first place. He wants to increase speed, not learn music theory. Plenty of amazing guitarists get by being musically illiterate. But I do agree with you, learning music theory helps a lot. And bad habits should never be practiced, even if it makes you worse for a little bit to stop them.
speed comes naturaly as you become confident in your movements
if I were you I would work on expressiveness while you are slow and let the speed come naturally and don't get to caught up in it once you develop it because it limits your ability to work on a givin note to create color within it