Frontend & browser·Performance and Core Web Vitals
you told the browser to skip rendering the sections that are off screen and the initial layout got much faster.
content-visibility
Also calledCSS containment, contain
A property that lets the browser skip style, layout and paint for a subtree until it is near the viewport. On a long document it can cut initial rendering time dramatically, because most of the page never gets laid out. It needs `contain-intrinsic-size` to give a placeholder height, or the scrollbar jumps as sections are rendered, and it hides content from find-in-page in older implementations.