The observation in the linked video is that, when the browser is incrementally rendering the page, a traditional "grid" layout will put the content in its final location, while a flexbox will render a box of content that's bigger than usual and shrink it later -- because the items that cause the content area to shrink haven't been loaded yet.

Neither do I. Unless flexbox's CSS weights a lot more than classical CSS, I don't know how could that be a factor to render slower. Does not make any sense.