As a collaborative pianist, I have noticed that when working with young players, their tempo tends to fluctuate depending on the difficulty of the music. While I can adjust my playing to compensate for this, it highlights the importance of effective practice. I would like to offer some suggestions on how to practice in a way that helps maintain a consistent tempo.
When you begin to practice a piece, know exactly what you need to practice. Our tendency is to practice what we want to practice. But you have to be completely honest with yourself.
Many players practice by starting at the beginning and playing through to the end. Eventually, you may eventually bring the piece up to an even level of quality and tempo, but it will be a long and error-prone process. It’s far better to start where your goals tell you to start. See above. If your goal is to get that tricky passagework on page 2 up to tempo, then that’s where you should start. Your progress will be much faster this way.
Almost any violinist can play one note of the Tchaikovsky Violin Concerto. With a little practice you could play that note, plus another at tempo. How about three notes in sequence? Four?
This idea hints at how you can bring any passage up to tempo. Start with two notes. Practice those at tempo. Now repeat, but adding a third note. Keep practicing that sequence until it’s even, musical, and at tempo. Now add another note. Keep going in this fashion until you’ve mastered the passage, or at least you achieve the goal you were striving for today.
If the passage is not too difficult, you could add notes in 2, 3, 4 or 8 note units.
Slow practice has its purpose - mainly to learn the notes. Once you know the notes, you need to quickly transition to learning the technical moves required to play the notes at tempo. Why is this? Prolonged practice as slow tempi encourages you to use slow muscle action. Consider fast passage work in a Vivaldi concerto. Yes, you can keep playing it at very slow tempi and gradually moving it upward, but you will find at some point that the technicals moves you were using at slow tempi no longer work. Maybe you were using long slow bows, whereas performance tempo makes that impossible (and musically incorrect, too.) Sure, practice slowly to learn the notes. Then transition early to the sort of additive practice described above.
All of the advice I’ve given is predicated on the use of the metronome. You simply must do it. When I’m preparing the accompaniment parts of your pieces, I’m constantly using the metronome. If you do too, we will make incredible music together!