Big pipe is new since drupal 8 and introduces a way to load and render blocks which have a high cache invalidation, or are uncacheable, during the stream of the response. This increased the perceived performance and feels like your site is very fast.
But this only works for block out of the box with auto placeholdering.

Main content block is excluded, so if you have a lot of paragraphs in your main content the load time can increase for uncached pages.