The Pareto Principle (80/20 rule) says that 20% of your efforts will produce 80% of the results. And many many things follow similar patterns.

It seems to be coming up in a lot of books I have read lately.

For saxophone, it might be the long tones you play for five minutes in your practice that account for most of the improvement in your sound.

Or it could be the two minutes you spend practicing the single difficult measure in a song that makes you sound a lot better overall.

Another way to get much more improvment is to simply pracice LESS.  Maybe practice 20% of the material that you used to practice.  And that smaller amount of material will become much better, strengthen your fundamentals, and therefore turn you into a better player faster.