Hey this is actually a really cool topic
Playing guitar fast and smooth requires two skills to develop: your fingers' 'muscle memory', and your brain's processing speed! It's actually quite important to train BOTH of those, but rarely do you hear mention of the brain. The focus is almost always on your finger's strength, speed, agility, calluses...etc.
I will show you exactly how to BLAST your way through any learning curve, using a few very simple techniques that train both of your two skills separately, and then together at the same time.
Relaxing your fingers:
First of all, it's important to understand that playing fast for long periods of time does NOT come from finger strength, speed of motion, or anything like that. It comes from how fast you RELAX your fingers after playing each note. Therefore, in this first exercise, you will train your fingers to play RELAXED! Here's what you do:
Take a long string of notes that you've already memorized (a scale pattern for example, or a song that keeps your fingers moving). Instead of playing this piece by pressing down on the strings, what you should do is just gently rest your finger on the string, WITHOUT pressing down to touch the wood! So instead of getting a sound, it should be a muted note.
Play with that muted-note technique slowly at first, then gradually build speed. You can continue to pick the notes that should be played, even though there will be no sound. It's as if you're playing invisible notes!
There should be very little tension in your hand, wrist, fingers, and body. You need to stay relaxed while you're doing this. It's CRITICALLY IMPORTANT! I won't go into details here because it's not the focus of this post. Just picture what would happen if you were doing anything while being tense. If you were driving you might slam on the brakes and hurt someone, if you're talking you might scream... just keep yourself at a very relaxed state, and do NOT try to go as fast as you can. That will mess up the whole process.
Play in that muted-note technique for about 10 minutes. You should already feel the effects. The next time you play guitar normally, it will be a little bit easier for you to play faster, longer, and without as much tension. Continue to practice in this way for several weeks.
Advancing players will eventually notice that they are able to play very fast in a small section of the guitar neck, say repeating pattern of 5 notes. However, as they try to blaze up and down scale patterns in various directions up and down the guitar neck, it becomes difficult to keep up the speed, and very easily they will get lost and forget where they are. This is normal.
Your brain is actually working VERY hard to visualize all the scale patterns that you've practiced for so long, and then trying to connect the scale patterns together, and then telling your fingers to do all this stuff... As you start to focus on building speed, your brain can no longer think about visualizing the patterns, and thus you will slow down. It's like a computer bogging down because there are too many processes running at once.
The solution to this problem is actually quite fun! It won't take very long, and you can also work on building speed while pushing your brain to think faster:
Basically, instead of playing a scale across 6 strings, you're going to play it across 2 strings but all the way up the fretboard and back down.
Pick any two strings on your guitar neck to work with. Play the first 6 notes of any scale that you know (that should use up the two strings you picked, 3 notes on each string). Move up one or two frets. Play the next 6 notes in that scale, using the SAME pattern.
It should look something like this:
A string: --------3-5-7-------5-7-9--------7-9-10----------9-10-12---
E string: --3-5-7-------5-7-8-------7-8-10--------8-10-12------------
Naturally this requires knowledge of how to play a scale up a single string. It shouldn't be too hard to pick out the notes by ear, and then tab them out on a piece of paper.
You should pick a repeating pattern, and go up and then back down these two strings just like above. Change patterns every couple of repetitions.
Just force your brain to think faster and faster by constantly shifting the position of your hand, and changing the starting position of the scale.
As you start to process these notes faster, you can add another string! This time you can play across 3 strings... and then 4 strings... and soon 6 strings will be no problem, and you can just hop around anywhere across any section of the neck anywhere else fluttering notes up and down the block.
I suppose it's difficult to explain this through words. I can possibly create a video if that will help more.