I recently got this question in a Facebook group:
"I’d love specific ideas for learning to play fast. I know how to 'work up a passage' but I'm looking for ways to in general get better at playing fast. Do I need to just continue working on the things I'm not as good at, or are there specific ways to work on speed in playing? I think its fun to play fast, but find it very difficult. I heard a ballet teacher say you have to dance fast to be able to dance fast... thoughts?"
Great question. Usually when students ask how to play fast, they get information on 'working up a passage,' which is a short-term solution to a specific problem. It's a valuable skill to have, but most musicians will quickly find that they hit a wall. Here's why:
Playing fast is actually based on a number of different skills and competencies, and a weakness in any of them will limit your ability to play faster.
The first is technical skill. You already know how to work up a passage, so I think you mean that you're having trouble playing NEW things at faster speeds. General technical skill can be built up by playing scales, arpeggios, and other technique exercises at faster speed. The metronome will help smooth out your technique and iron out little problems that cascade into bigger problems when you're playing fast.
The next major problem that people have when playing fast is problems processing the information as quickly as it's coming to them. That usually means that you're processing information in chunks that are too small (individual notes instead of patterns like arpeggios and scales). To improve your processing, you’ll need to improve your mental model and make it more automatic.
Your mental model is your understanding of how the different components of music fit together. An understanding of scale degrees, harmony, and rhythm are all parts of the mental model. Luckily, a big part of developing a mental model is just getting more familiar with your scales and arpeggios, so those do double duty. Another part is putting those technical skills into a larger number of musical contexts. You can practice that by sightreading a lot more music. Etude books are great for that.
It can also be very helpful to expand and strengthen your mental model (the way you process music and make predictions about what is coming next) by sight-singing often and listening to more classical music (I mean the broad use of ‘classical’ here). Most mental models improve with experience, but there's no reason to wait years to let it improve itself when you can speed the process along with a little focused effort. Good luck!