I haven't done any real analysis of it, but I've always assumed it's because it renders up a single small HTML document rather than large core content and dozen

I haven't done any real analysis of it, but I've always assumed it's because it renders up a single small HTML document rather than large core content and dozens of external references (ads, analysis, etc.). Looking at this page in my browser's network analyzer, I see a request for the page itself (3.8k), followed by requests for the css and a few images which don't impact layout. On top of that, JS isn't used for layout, so the HTML renderer can do its job.