There’s a quote that’s popular at the moment: If you always sound good in the practice room, you’re probably not doing it right. I kind of agree, but the converse is true for sure: If you always sound bad in the practice room, you’re probably not doing it right.
The first quote is getting at something important: if I practice what I already know, I may sound great, but I won’t learn anything new. If I practice stuff I don’t know, maybe I’ll make mistakes (that’s OK) but to me it’s important to sound good even while I’m making mistakes. Mistakes sound good if played well.
For me the purpose of practice is to create good habits, and to refine existing habits. The aim is to be able to play something that is beautiful, fluently, with good time, sound, expression, and feeling, and to do these things without thought. If I practice something that is too difficult, I find myself struggling to play it, and that struggle gets built into the habit that I create, along with poor sound, time, and expression. I am practising struggle rather than fluency. And that’s what comes out on the gig.
When that happens, I break it down and slow it down. There are few things that are worth playing that remain difficult to play when slowed down enough. If I still can’t play it at quarter speed, then maybe I should put it to one side for few months and meanwhile try something simpler. When I can do it slowly but well, I gradually speed it up. This process can take a long time, months, years, or maybe is never complete.
For people learning jazz it’s easy to get the impression that it has to be difficult. It’s hard, but that’s not the same thing. Hard because it takes such a huge amount of work to get all that stuff together. But each step on the way should be within your grasp. There’s always a temptation take on stuff that is too difficult. Jazz is complicated music, in every department. As a learner you need to find ways of simplifying it, to find something that’s relevant that you can play well. You can play quarter-notes or half-notes instead of eighth-notes. You can play V-I or V or I instead of ii-V-I. You can leave a lot of space. You can just play the tune and decorate it. But make it sound good.
There’s always a temptation to play fast. There’s no particular musical merit in this, and it can turn into the musical equivalent of being a boy-racer. Like Jim Mullen says of John McGlaughlin it’s like he’s terrified of playing a long note. If you’re 18 years old and speed matters to you, that’s fine. And if after years of study you develop enough technique to play fast where it’s musically appropriate, then that’s fine too. But as a learner you can play perfectly good music without playing fast, even if you aren’t world class. Just don’t let emulation of world-class players destroy the beauty in your own music.