[ … ] even if the browser is busy rendering the page or loading in content, the user should still be able to scroll around and interact with it.
Animations need to be silky, scrolling must be buttery-smooth and your page needs to contain little to no jank. This comes down to offering an experience that can run at 60 frames per second even on regular pages, not just games and animations.

To hit 60fps, we sometimes need to go beyond JavaScript as the sole performance bottleneck for our pages and spend more time investigating paint and layout issues styles might actually be the core cause of our sluggish performance.
In this post, Im going to talk about optimizing paint times for your pages.