MOUNTAIN VIEW, CA: Facebook is introducing "prefetching" in order to speed mobile ad load times and has warned advertisers that if their mobile website is slow their ads might not be shown.

This process involves "pre-loading mobile content in the Facebook in-app browser before a link is tapped", Facebook said.

"This can shorten mobile site load time by 29% or 8.5 seconds, improving the experience and decreasing the risk of site abandonment."

The threat of premature departure is one that advertisers cannot afford to ignore – Facebook's research suggests that 40% of users aren't prepared to wait any longer than three seconds for a page to load.

People tapping a link only to move on before seeing a site also raises issues around measurement – analytics firms may not register visits – and makes it harder to track and optimise ad performance.

Facebook may also opt not to show ads if the website it leads to is too slow and not optimised for mobile, said Matthew Idema, Facebook's director of ad products marketing.

"The speed with which a mobile website will load is one factor our ad delivery system will use to determine which ads to show which people," he told Advertising Age.

Search Engine Journal raised the possibility that advertisers with slow-loading websites could end up paying more.

"The big goal here is increasing ad performance and engagement," it said. "If you have an ad that takes users to a slow-loading site, that ad will most likely underperform in terms of engagement, which means you'll pay more."

Mobile-optimisation jumped to the top of the digital agenda in April last year, when Google made mobile responsiveness a key ranking criteria in search – meaning that publishers with non-mobile-optimised websites were in danger of seeing their search performance suffer.

But while leading media sites have adapted and are fully aware of the need for speed on the mobile web, it seems not all advertisers have yet grasped this.

Jarrod Dicker, Washington Post's director of ad product and engineering, explained that site users had been trained that if an experience isn't fast it's OK to leave or to bounce".

"Advertisers need to take their creative and their message and make it as fast as possible," he stated.

Data sourced from Facebook, Advertising Age, Search Engine Journal; additional content by Warc staff