Ever wondered why an ad block on a website takes so long to load, given that advertisers are forced to ensure that their creative meets max file size restrictions to ensure quick loads?
Well, the answer may be in how the ad placement firms that publishers engage to fill ad slots use real time auctions to determine which ads to display.
When a web page is loaded, the ad slots on the page ask the ad placement company for ad creative. When they do, the ad server runs a real-time auction amongst potential buyers. This auction is supposed to wrap up within ~100ms or so but, in an effort to maximize profits, they may ignore this time limit in an effort to retrieve potential bids from more advertisers, driving up the price.
Any website page that doesn’t run async scripts would experience extra long load times and a severely degraded end user experience.
Now, it is common to see a web page built in without ads, and then have those ads delivered, often adjusting the dom in the process, resulting in a web page that never stops loading.
On older devices, like the crappy original iPad I have at home, this will often cause a browser crash, the result of a dom or script conflict, as page elements appear, vanish and appear again during the dom rebuilding process.
On newer devices, web pages load in a jumpy fashion as each ad block builds into the display, shifting content around and causing havoc.
Anyways, Business Insider had an interesting article on this yesterday: http://www.businessinsider.com/conspiracy-web-pages-load-slowly-because-they-make-more-money-that-way-2015-7