Ad blockers can both speed up and slow down mobile sites, a new study from digital performance monitoring company Catchpoint Systems Inc. finds.

Catchpoint’s study examined how the Pi-Hole mobile ad-blocking software impacted mobile site load times in May. Catchpoint defines web page load time as the time it takes for enough of the page elements to load for consumers to start interacting with a page.

Mobile ads typically delay the time it takes for a consumer to be able to start interacting with a site and they also eat up a consumer’s data, spurring a recent rise in ad blocking popularity. Consumers download ad blockers not only to avoid seeing ads, but also to be able to more quickly navigate sites.

In the study, Catchpoint monitored 20 financial services, retail, travel and news mobile sites. In most verticals site speed increased with the ad blocker. However, on average, financial services mobile sites loaded 12% to 38% slower when ads were blocked. In fact, seven mobile sites in the sample slowed with ad blocking.

“You can no longer assume a mobile site will always load faster with ad blocking on,” says Mehdi Daoudi, CEO and co-founder of Catchpoint systems.

Mobile sites that display numerous ads, such as news and retail sites, did load more quickly with ad blocking. Financial services sites typically have few ads, so the time it takes for the site to interact with the ad-blocking software is greater than the speed improvement the site would have gained from blocking ads, Catchpoint says.

“While our findings show that advertiser-dependent sites tend to load much faster with ad blocking engaged, there was a surprising number of sites whose user experience actually got worse,” Daoudi says.

In addition, some sites can detect when an ad blocker is on and then display a pop-up message alerting the consumer that ads are blocked. Displaying this message can slow down a site more than ads do, Catchpoint says.

Financial services sites that slowed with ad blocking include Chase.com, which loaded in 4.53 seconds with ad blocking and in 3.27 seconds without it; BankofAmerica.com, which loaded in 5.27 seconds with ad blocking, 3.84 seconds without; Citibank.com, 4.59 seconds with and 3.81 seconds without; and WellsFargo.com, 3.79 seconds with, 3.39 seconds without.

The other mobile sites that slowed with ad blocking are: Southwest.com, which took more than three seconds longer to load with the technology. It loaded in 7.28 seconds when ads were blocked, compared to 4.16 seconds when they were not; Huffingtonpost.com, 4.39 seconds with blocked ads, 3.59 without; and Amazon.com, 1.68 seconds with ad blocking and 1.64 seconds with ads displayed.

On average, news mobile sites loaded 27% to 48% faster when ads were blocked than when they weren’t. Travel sites loaded 19% to 51% faster, and retail sites loaded 9% to 14% faster.

Mobile sites that improved the most with an ad blocker include CNN.com (7.60 seconds compared to 14.76 with it off), Hotels.com (4.31 seconds compared to 8.86 seconds) and NewYorkTimes.com (4.79 seconds versus 8.21 seconds).

Consumers can block ads several ways on their smartphones, such as via an ad-blocking browser, configuring the browser to block ads or downloading an ad-blocking app. Pi-Hole, the ad blocking software Catchpoint used in this study, uses domain name system manipulation as its ad blocking technique. This technique blocks ads at the router level, Catchpoint says.

About 419 million consumers, or 22%, of the 1.9 billion global smartphone users have installed an ad blocker, nearly double the percentage of a year ago, according to the “2016 Mobile Adblocking Report” from PageFair Ltd., a company that works with web publishers to fight ad-blocking software.