Why do some 'Jazz pianists' insist on playing fast? Maybe it's cool? They like to show-off perhaps? Maybe they are under the illusion that playing fast represents great skill and musicianship?
Well, it's all bollocks.
Playing fast means you can play fast. Well done, enjoy this cookie. What it also does is cloud our awareness of what music really means and what people have come to listen to you for.
Speed is like red wine; it's good in moderation, but too much makes you sound like a prat.
Constant speed is usually a technique employed by people who are unaware of the fact that music is just an audible representation of what we feel inside. To know this simple fact is one thing, but being aware of it is different. How can we sound really good without needing to hide our lack of passion and musical connection with speed?
Silence. Space. Self-control. Purpose.
Silence: Stop playing sometimes. Let the rhythm be the canvas onto which you strike your musical brush. An artist who never stops painting in the sense of never letting his brush leave the canvas but to re-ink his tip, will most likely be very unaware of his progress. He will miss little details, more than likely stumble or place an incorrect colour first when it should be second. His observational necessity will become clouded and he will no longer see his work for what it really is. At the piano, be aware that silence is also a note. A note of intent. You stopped playing because it felt right; because you are taking some time to process progressive internal feelings as the song passes and as your thoughts are churning over various aspects of your life that the song reminds you off. So, just stop sometimes. Allow your left hand to simply lay down the simple rhythm, or if in a trio, enjoy the bass and ride symbol for a few bars. Enjoy the ride instead of smothering it with endless nonsense.
Space: Similar to silence but more widespread and structural. Where silence refers to bars and momentary pauses, space refers to playing shorter passages (which could be fast or slow, depending on how you feel). Remember that filling a canvas with colour ends up black if you don't stop. A meaningful phrase here, an interesting melody there, a spot of block chords here and a bit of parallel hand melody there. Space is the breathing out of the lungs. Fill, release. Don't keep filling or it will become nasty and ridiculous. Whoever wanted to hear or see that?
Self-control: Put a nice sugary sweet in your mouth and don't bite into it. If you can do that, you have a good self-control. You had a goal set and you achieved it by, for want of a better word, 'forcing' your body to do what you tell it. Being at one with your body is very important for playing the piano well. Your fingers, you might think, are controlled by your muscles and voluntary actions. Biologically speaking, yes, but a deep understanding of music and improvisation knows, shows and proves otherwise. When in control of your thoughts and voluntary movements, art is produced in a much more natural, meaningful and more importantly, personal way. Sit at the piano and do not blast out scales. Take your time and understand that you're producing something special, personal and unique that nobody else could do in the same way as you. They could never reproduce something in exactly the same way as you because it would be unnatural and false; likewise, you could never copy another so don't bother trying.
Purpose: The most important of all, so the last thing you are reading, thus you should remember it most. When you sit at the piano, ask yourself this question: Why am I playing the piano right now? You might choose a particular song to play. Why this song? Did you hear it somewhere and wanted to play it (by sight or because you have it in your repertoire)? Play a song with purpose, and you will play well. Scaling down this concept, why are you playing this chord? Why did you choose to improvise using that scale? If your answers are "because I know them", or "because it's my favourite scale", then you are not playing for a purpose, you're just playing because "You can". So what? Lots of people Can play the piano or draw a wonderful picture. Why should anyone care that you can do it, too? Maybe that seems too heavy or blunt? Good, it should be. When you sit and play, having an understanding of why you are playing a song, a melody, a scale or a chord, or even using that particular fingering, is important. This doesn't necessarily mean that you are to analyse what you are playing at every moment, because thinking too much always blocks natural feelings and output, but understanding why you are to play it is so invaluable as to its purpose in your performance. An artist could explain the reason for using a particular brush or tone of colour, even though at the moment of brush-to-canvas, he was just following his natural purpose. Understanding such devices available to you enables you to produce more deep and meaningful music which people will hear in your performances.