Playing with reliable speed and accuracy can be achieved by anyone; but to acquire this ability in the least time possible, takes considerable skill at the art of practising and a lot of self analysis. It helps to make some mistakes too - fall over, pick yourself up.
Playing fast usually means shredding with single notes through scalar phrases, arpeggios and riffs. Chord changes can also be difficult to get right at speed but that's another story.
Only after you have spent time truly learning and memorising a phrase, does it become possible to speed things up. This can be done methodically by starting with simple practice techniques like 'dotted notes' Staccato and 'walk-run-walk' - these will help you nudge up you're speed gradually. If you've ever wondered why classical guitarist have such great technique and can play fast and clean so effortlessly, all you need to do is look at the methodical learning material used to train them. It is easy to find these two ['dotted notes' and 'walk-run-walk' ] speed building techniques in all the studies and exercises.
Dotted 8ths are very commonly used across music styles, but Classical music doesn't stick to a beat like Rock, Jazz and Blues. It often slows down, and speeds up using Rallentando and Accellerando - which translates to walk-run-walk when you practise a scale or lick. I'ts very useful and effective.
Staccato is an articulation in classical music that when translated to guitar means you have to mute the string immediately after picking the note. If you practise scales with staccato and use your pick to mute the note, this trains your fingers to be more economic with movement and stay closer to the strings. Over time this unconscious economy of movement means you can play faster.
I show this Staccato technique in a video I made for the Rocking Legato interview here.
If you find that your technique breaks down when you speed up, then you really need to find out specifically why that is happening - realise which techniques are failing you when you accelerate. This is where the practicality of 'Blooms Taxonomy' make sense.
Imagine your bicycle has a very slight buckle in the back wheel. When you ride normally you don't notice it, but when you race fast downhill, the ride becomes unstable and your tyres lose traction on the road, slowing you down. You can still ride quite fast, but no faster, because you have a buckle in the wheel. This buckle is so slight that you can't see it just by looking at the wheel.
It;s the same with your guitar technique; you have to find the subtle flaws, the weaknesses that expose themselves in certain situations - and drill them out methodically.
You need to examine every movement or action that your body, hands and fingers make while you play. As soon as you become aware of what you are actually doing in slow-motion, you will have begun to fix it.
There are 5 main considerations:
Picking hand efficiency
Fretting hand efficiency
Left and Right hand coordination
Body Tension + Breathing
Multitasking and micro-time
OK, let's say you've learned a riff from a YouTube lesson, but you can't play it fast enough. You have to play it at a gig in 3 weeks time, what are you going to do?
What you need to do, is find out specifically why you can't play it faster, and work on those weaknesses in your technique - one by one.
But you can't really examine your own technique until you fully understand the 4 considerations. I'll explain them as briefly as possible.
Assuming you hold a pick (plectrum) to pluck the strings you have to look at how much movement is going on when you pick the strings.
For example, lets say you want to alternate pick every note of the phrase, then look at how much your hand moves around between upstrokes and down strokes, and between adjacent strings. if you need to play 16th notes at 144 bpm, then you'd better keep your fingers as close to the strings as you can.
This comes down to how far your fingers are away from the strings while you play the phrase. Basic physics applies - the bigger the distance your finger has to move, the longer it takes.
Everyone has a speed limit - the speed at which the accuracy starts to fade. Lets say your picking speed is fast already, and your fretting hand is very efficient too, but you still can't seem to get past 16th notes at 120 bpm without messing up. This might be down to the timing between when you pick and when your left hand is on the right note. ( We are still assuming alternate picking here)
I demonstrate this concept on the Speed Learn A Riff page.
Some years ago while I was studying and performing jazz, practising for 4 or more hours daily, I became aware that I'd unconsciously begun holding my breath during some of the more difficult fast passages that I struggled to play accurately. It took me a little while to learn to stop doing that but as soon as became aware that was holding me back, I was able to get over it and progress.
This tension in my breathing was also preceded by a stiffening up in my arm and shoulder which seemed to get worse the faster I played. It's very easy to let this kind of body tension spoil your speed development and hold you back for years, if you fail to look out for it and eradicate.
No-one can truly multitask - not even women ( so the new science tell us). If you slow time down into milliseconds or micro seconds what actually happens is just one movement at a time, which, when sped up, looks and sounds like multitasking to our ears and eyes.
Check the order of actions - left hand moves into position, right hand pick sits on the string in prep, pick plucks note and comes to rest on same string in prep for upstroke, left hand index finger moves to new position, and so on.
Put all your actions in very slow-motion and watch them. This is something I explain better as part of Bloom's Taxonomy - Analysis.
Play a riff or phrase you know well with a pick. Have you ever noticed that you always start this riff on a down pick stroke? Have you ever tried to play it starting on an upstroke instead? Is it still easy?
This is one of the most commonly overlooked technique faults. Successful guitar players always strive to overcome weaknesses and get better and better. The goal has to be ambidextrous picking that doesn't rely on the easiest routes all the time.
One of the best and most well know examples of this is Paul Gilbert's 'Technical Difficulties' rock guitar exercise. This exercise tackles picking technique - particularly the difficulty of crossing adjacent strings by alternating the inside/outside upstroke/down-stroke crossover.