Third Party Web: an analysis of third-party script costs

Third Party Web is an initiative to analyze the cost of third-party scripts on websites to provide Internet users and developers with actionable information.

The project has four simple goals:

  1. Quantify the impact of third-party scripts that run on the top 1 million sites according to Alexa.
  2. Identify scripts that have the greatest performance costs.
  3. Provide developers with information.
  4. Incentivize responsible script behavior.

The project team runs two scans on about four million sites per month using Lighthouse on mobile to identify third-party scripts and the performance impact they have.

According to the data, about 65% of all script execution time is caused by roughly 800 origins; the top 100 origins account for about 59% of all script execution time on the analyzed sites.

Read also:  Extensions may impact performance.

The project sorts scripts into categories such as ads, social, analytics, or video to make it easier to compare the performance impact of related scripts.


The biggest offenders in regards to the average performance impact:

  • CreateJS CDN — Libraries — 3188ms on average
  • WordAds — Advertising — 2543ms on average
  • Popads — Advertising — 1245ms on average.
  • 33 Across — Advertising — 1170ms on average.
  • Wix — Hosting Platforms — 1153ms on average

All scripts, with the exception for the Wix script, which was found on more than 158k sites, were found on a relatively low number of sites (about 30,000 or less).

The biggest offenders in regards to distribution:

  • Google/Doubleclick Ads — Advertising — 1412404 executions, 330ms average.
  • Google Tag Manager — Other — 1093167 executions, 386ms on average.
  • Wix — Hosting Platforms — 158466 executions, 1153ms on average.
  • Facebook — Social — 1212567 executions, 120ms on average.
  • Google CDN — Libraries — 811231 executions, 178ms on average.

Advertisement and Mixed/Other scripts make up the largest chunk of third-party script executions.

It should not come as a surprise that scripts by major Internet companies — Google and Facebook specifically — are found on a large portion of scanned sites. Google alone has five scripts in the total impact top ten, with three of them breaking the one million execution barrier.

Google scripts were found in about one in three sites at a minimum, Facebook scripts in one in four sites.

Closing Words

Third-party scripts impact web performance significantly; a simple visual comparison of the loading performance of sites with and without content blockers is enough to highlight that fact.

The result of the study is not really that surprising: third-party scripts impact performance, and Google and Facebook have scripts running on a large portion of Internet sites.

Now You: What is your take on the study?

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