• by Laurie Sullivan  @lauriesullivan, June 22, 2016

Mobile Web page load times have become a focus as brands look to optimize sites and content through Google's Accelerated Mobile Pages Project (AMP), but a study released Wednesday shows some ad-blocking technology can slow the load time of certain Web pages -- in one instance, by as much as 3.12 seconds.

In fact, Southwest Airlines’ mobile site loaded in 4.16 seconds with ad blocking off, slowing to 7.28 seconds with ad blocking on, according to a study released by Catchpoint Systems, which collected data from May 1 to 31, 2016 to analyze page-load times.

The study monitored 20 mobile sites using Pi-Hole DNS-based ad blocking. Tests were run at 15-minute intervals and results were collated into a median average for each site.

These are significant slowdowns as businesses grapple with increasingly impatient consumers. Google research indicates that differences of 250 milliseconds -- one-quarter of a second -- influence people to bounce from one site to another never to return.

eMarketer released a forecast Tuesday suggesting that 69.8 million in the U.S. would use ad blocking technology this year, up from 34.4% in 2015. Next year, that figure will rise 24% to 86.6 million people.

Ad blocking is more common on desktops and laptops than on smartphones, says eMarketer. This year, 63.2 million people will use an ad blocker on their desktop or laptop PC, vs. 20.7 million who will use one on their smartphone. About 90.5% of ad blocking users will block ads on desktops and laptops, while just 29.7% will do so on smartphones.

Mobile travel sites, excluding Southwest, saw significant performance improvements when ad blocking was turned on, with most of the sites reporting improvements between 19% and 51%.

Sites that are not typically ad-heavy, such as financial services, slowed the most with ad blocking engaged, although they had less content to load. Chase Bank’s mobile site loaded in an average of 3.27 seconds with no ad-blocking technology, but with ad blocking it slowed to 4.53 seconds. Overall, most financial services sites slowed with ad blocking activated, becoming 12% to 38% slower than with ad blocking off.

Interestingly, the study found that ad blocking, as intended, did produce faster load times on most mobile retail sites, and had an even larger performance boost on news sites, which are normally the heaviest and most ad-intensive. In some cases, however, the study found that non-advertising files were blocked, delaying the start of page rendering.

Catchpoint CEO Mehdi Daoudi explained that in some cases publishers will install counter measures to ad blocking. Programmatic technology is another reason why some Web pages in certain market segments might load faster then others when the individual uses ad blocking technology. "Some publishers use first-party domains," he said. "But you see the impact more when the site relies on programmatic ads where many redirects occur or when the site relies on third-party ad servers." 

The study also found that some news site Web pages, such as the Huffington Post, slowed with ad blocking on, while CNN's mobile Web pages improved in speed load times.