There are many musical pieces that seem almost impossible to play because they may be too fast and/or technical at your current stage of guitar skill development. I used to become frustrated whenever I tried to play my fastest because it would only be a little more than half the speed of the song I was covering. Little did I know that I was doing it all wrong. Guitar speed is not something you could gain overnight, nor after a long day's practice. Building speed takes a lot of time and even more patience. Follow the steps correctly and you WILL be playing much faster by the end of the week.
A metronome is not required but is highly recommended.
STEP 1. Determine what you will be playing
STEP 2. Play it as slow as humanly possible.
SLOW. Very slowly. So slow that it doesn't even make sense anymore. Play it as comfortably and smoothly as possible, utilizing hammer-ons, pull-offs, and proper picking technique. Be sure to keep your left hand (if you are a right handed guitar player) as close to the strings as possible when playing. THERE SHOULD BE NO PAIN WHATSOEVER. About 20% of the song's bpm should do.
Step 2 is crucial because you are building muscle memory for your hands. For this step to work to it's maximum effect, you must practice this step for about 5-6 hours and play nothing else in between. I know...patience...It doesn't have to be all in one day. I spanned the 6 hours over a week.
STEP 3. Evaluate your playing. If you are playing at the slowest speed PERFECTLY, then you can move on. For step 3, you can play at about 25%-40% faster. For example, if you were playing at about 50 bpm before, 65-70 bpm is where you want to be playing for this step. Practice this for about 3-4 hours, with complete relaxation and economy of motion.
STEP 4. If you can play step 3 perfectly and smoothly, without pain, begin to play at about 50%-65% of the speed of the song. For a 250 bpm song, you should play around 125-165. It is okay to slowly build your speed from 125-165 bpm (just an example). Practice this for 3 hours.
STEP 5. After you mastered step 4, play at about 75%-80% of the song's speed. If you can't, continue practicing step 4. THERE SHOULDN'T BE ANY PAIN. DO this for 5 hours.
STEP 6. 85% speed for one hour.
STEP 7. 95% speed for 10 HOURS! I know. Hard work.
Step 8. 100% speed for an entire week. Kill it.
Although long and challenging, this is a promising way to increase your guitar speed. Even if you can't reach the later steps, you'll find that whatever you were playing should feel much more natural than before. I hope I help some of you.