What is layout, paint and compositing? Google explains
Users notice if sites and apps don’t run well, so optimizing rendering performance is crucial.
Performance is the art of avoiding work, and making any work you do as efficient as possible. In many cases it’s about working with the browser, not against it. It’s worth bearing in mind that the work listed above in the pipeline differ in terms of computational cost; some tasks are more expensive than others!
Read the paper: Web Fundamentals