Ads are responsible for making webpages slow to a crawl, suggests analysis of the most popular one million websites.
The research by developer Patrick Hulce looked at which chunks of code take longest to load.
About 60% of the total loading time of a page was caused by scripts that place adverts or analyse what users do, he found.
But using ad-blockers may not be the best way to avoid delays and speed up the loading of webpages, he said.
If used together on a site, these can add more than two-thirds of a second to loading times, he found.
The worst offender was the WordAds script for WordPress blogs that, all by itself, can delay the arrival of an entire page by up to 2.5 seconds.
Not all delays were down to ads and analytics, said Mr Hulce. Other factors such as network delays and big file sizes for some content could also contribute to slow loading times.
In addition, he said, greater use of ad-blocking programs may not always improve browsing speeds.
Ad-blockers can end up "triggering convoluted workaround logic and complex disguising of ads that increase script execution time", he told The Register.