In brief
The browser can postpone some of the drawing out of sight.
Useful to know
content-visibility: auto allows the browser to skip work on shaping and drawing content when it is not relevant to the user. When approaching it, the necessary work is done. [1]
This is different from deleting HTML or refusing to load the data. For example, a long list of sections can benefit, but the result depends on the content and the browser. Think of a suitable spare size through contain-intrinsic-size, to limit jumps in the arrangement. Measure the effect on a real page instead of taking the universal acceleration property.
Sources
For further detail and to check the information: