NEW YORK: Time-pressured internet users often say they install ad blocking software to speed up their browsing experience, but they may be surprised by a new study which found ad blockers can actually slow load times on some mobile sites.

Catchpoint Systems, a website performance company, analysed load times on the mobile sites of 20 leading brands, operating in the news, retail, financial services and travel sectors.

The study monitored the time it took for content to load with ad blocking software switched off and then compared the results with the time taken with it installed.

Although load times speeded up for the majority of brand websites when ad blockers were switched on, performance worsened on seven mobile sites, especially for brands in the financial services and travel sectors.

"Here's a surprise, though: sometimes it's not ads that hurt performance, but ad blockers," said Mehdi Daoudi, CEO and co-founder of Catchpoint Systems, in a blog post.

He added the caveat that the report analysed data only from Pi-Hole blockers, meaning the results could differ from AdblockPlus and other popular software, but he said: "The point is still valid: you cannot assume that ad blocking will always improve site performance."

For example, Southwest Airlines' mobile site loaded in 4.16 seconds with ad blocking software turned off, but it slowed to 7.28 seconds when it was switched on.

Chase Bank's mobile site had an average load time of 3.27 seconds with ad blockers disabled, but this slowed to 4.53 seconds when they were activated.

Indeed, the mobile sites of financial services companies performed the worst when ad blockers were enabled, with most sites becoming 12% to 38% slower.

By contrast, mobile news sites saw the best improvement in web performance – an average increase of 27% to 49% – after the installation of ad blockers.

But with news sites often heavily dependent on advertising for their revenue, the findings also revealed that they were the ones to suffer the most when ad blockers were switched off. Publishers would have to weigh up the benefits of faster load times against their ability to serve ads.

The average load time on CNN, for example, was 14.8 seconds without ad blockers installed, but the time taken to load one of its pages dropped to 7.6 seconds with them activated.

Data sourced from Catchpoint Systems; additional content by Warc staff