So we all know that scales are ok but once you get to a certain speed then it's like your body hits a wall and it's physically impossible to move that fast.
I learn't this technique when watching a John Petrucci guitar video. The trick is to start off slow. And I mean slooooooooooooooowwwwwwwwwwww.
Basically what you are doing is programming you brain. It's tricking yourself into doing it. If you try to sit down, set the metronome at 200 bpm and try an a major scale for the first time, then most people won't be able to do it.
So here's what you do....
Set a metronome. For 60 bpm. Yes. That's right. It's ridiculously slow. You'll feel stupid for doing it but believe me it's all for your own good.
Now drill this for a few octaves up and down. Now here comes another secret. CONCENTRATE. Think about what scale degree you are hitting and with what finger. Keep it really steady and also keep the dynamics CONSISTENT. This means hitting all the notes with the same strength and length. One of the most important things to remember also, it to keep your fingers RELAXED. It might seem like to go faster, that you have to use more muscles and play harder. It's the opposite. you can only go faster when you have control of your hand. More muscle and power is used for dynamics NOT speed.
Then you raise it 5 bpm to 65 and do the same. Yes it's boring, yes it takes a long time. Just do the thing. it will make you better I promise.
Once you are comfortable with that then raise another 5 bpm. You will fin that as long as you are concentrating, then it will progress steadily and easily. Certain parts of the scales can be harder than other, like when you are using a fourth finger transition. What going slowly allows you to do is have time to know that the hard bit is coming up. Once you master it at a certain speed you are ready to go up again. You are giving yourself the ability to think faster.
If you still hit a wall then that's ok. If you are starting out, you probably can get maybe up to 160-180 ish before your brain says no. There are two things to do at this point. 1) Go to sleep. Have a nice long sleep and let your brain process all the information you have learned. Neural pathways become more permanent after a sleep. 2) Go back 10 bmp. What was hard 10 minutes ago before you mastered it by concentrating will now seem so easy. Repeat the process.
I'll try and film a video of me doing this in a key I have never learned ( I haven't tried the flat majors yet). and I'll show you how it works.