When you visit an external site like this (at ScienceDirect), you see that the paper is loaded progressively. Initially it just shows an abstract. Then, as you scroll down, it loads the next sections, one by one. I reckon such feature could be useful to some Wikimedia projects with large articles (Wikipedia, Wikisource, at least). Is there an extension which does this?
Topic on Project:Support desk
Well, there are many technical considerations that prevents an easy implementation, such as the fact that an entire page's source is a single entry in the DB, and that there should be a proper fallback if the user's Javascript is disabled. Even if it were possible, such an effort is not worth it to save a few - at most - kilobytes of text.
Is this a readability issue or an "I have a slow connection that I can't quickly load the article" issue? If it's the former - it could be simulated by hiding the text, but only after loading the entire article.
Such system could be build into Extension:MobileFrontend, which hides sections by default.