When you use a metronome you are setting deadlines for when new information must be called up from your memory.
Start at a speed where you can play “it”. Whatever it is, a song, a phrase, and exercise. Start at whatever speed is easy, and and set a metronome at the speed.
Play “it” at that speed a few times, to get comfortable.
Then increase the speed of the metronome a little. Maybe 5 BPM (Beats per Minute) Your brain now has slightly less time to deliver the same amount of information.
Play “it” a few times at this speed and then increase the tempo again, maybe 5 BPM. You’re slowly asking more of yourself. And at these tiny increments, you’re setting yourself up to be successful many times.
If you continue this process then eventually you’ll find a speed that is too fast for you. Your brain has reached it’s limit for how fast it can recall that information.
Note that your hands are capable of moving fast enough, just not with accuracy. That’s actually your brain, not your hands. Your brain doesn’t move slowly. It sends signals the same speed every time. But it needs to access the correct information from your memory. It needs to know exactly where to go. So really we’re training our brains to go to a certain place, with certainty and confidence.
If you repeat this process on a regular basis, you will be able to play anything you want. Play it as slowly as you need to, and then gradually increase the tempo. That might mean really slow. For me it does, I learn everything extremely slowly first. I believe learning material very slowly is the fastest way to attain mastery of that material. Haven’t we all wasted enough time trying to play things faster than we can?