Allegedly, on or about 22 March 2017, stan sent:
One thing I know slows down browsing is the way sites outsource much of their content. The browser has to access many sites on the web to put together a page for you to view. Blocking this as much as possible not only speeds up page loading, but also hinders tracking sites.
Though, occasionally, you get the opposite: Sites waiting for some external content to load before anything will proceed. Script blocking can help with that, but can also kill a site from doing anything. It's very much a two-edged sword.