Core Web Vitals and Ads: Why Publishers Should Be Concerned | Adzerk

Core Web Vitals and Ads: Why Publishers Should Be Concerned

Chris Shuptrine
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In May Google announced major upcoming 2021 changes to its search ranking algorithm, including the addition of Core Web Vitals, which are new metrics related to page load times and the jumpiness of content.

These new Core Web Vital scores include Cumulative Layout Shift (CLS), Largest Contentful Paint (LCP), and First Input Delay (FID).

Cumulative Layout Shift measures the stability of the page load: do the content and images move around before the page finishes rendering? Google recommends a CLS score of no more than 0.1 (using an equation based on content size and distance moved).

Largest Contentful Paint looks at overall loading performance of the main content. Does everything load within the recommended 2.5 seconds?

First Input Delay analyzes how network requests slow down the main thread - i.e., do slow-to-load requests prevent the user from clicking on a link? Google recommends an FID of less than 100ms.

Why programmatic publishers should be aware of Core Web Vitals

One byproduct of programmatic ads, especially with complex header-bidding setups or an overloading of ad units, is slow-to-load, jumpy content. Publishers generally accept this suboptimal user experience because of the ad revenue that comes with it.

Such ads, however, appear to have noticeable, detrimental effects on the Core Web Vital metrics.

If ads lead to poor Core Web Vital scores, then it’s possible these slow ad experiences could do more than frustrate users: they could also hurt SEO efforts.

And with Google search being such a heavy traffic driver for many sites, even a small dip in rankings could have a sizable impact on top-line revenue and page views.

It's imperative, then, that publishers are aware of their ad set-ups and not employing an ad strategy that could end up lowering their revenue in 2021.

How much do ads hurt Core Web Vital scores?

This is easy to test with the Google Chrome browser and its built-in Lighthouse dev tool, which will refresh a page and return two of the three core web vital scores (CLS and LCP).

core web vital programmatic ads
core web vital programmatic ads

To see how ads influence these metrics, I ran the report with and without an ad blocker enabled.

I did this across four high-volume sites, all of which show multiple programmatic ads on their homepages, including a top-of-page leaderboard ad. I ran the Lighthouse report multiple times, with and without an ad blocker, and below are the average scores.

  1. Cumulative Layout Shift: this looks at the jumpiness of the page load. The ideal score is < 0.1, using an equation based on content size and distance moved.
Cumulative Layout Shift ads
Cumulative Layout Shift ads
  1. Largest Contentful Paint: this looks at overall load speed of the main content. Target score is < 2.5 seconds.
Largest Contentful Paint ads
Largest Contentful Paint ads

These results suggest that programmatic ads do indeed hurt Core Web Vitals scores.

For instance, while three of the four sites were still above the 2.5 second target without ads, when ads were enabled LCP increased by 1.5 seconds on average. In other words, by enabling top-of-page ads on their site, these brands saw their LCP scores jump (aka, worsen) by 30%-45%.

It was even more stark with Cumulative Layout Shift. All four were within the recommended score without ads; all failed with them.

This is a very small sample size, of course, and the test quite unscientific, but the results nonetheless suggest that ads do decrease Core Web Vitals scores, which, in turn, could impact search rankings in 2021.

What can publishers do to prepare?

It’s important not to overreact here. Google has already stated that quality content supersedes these metrics, and we don’t know how much weight PageRank will give them (it could be close to zero).

But if it’s proven that slow ads do hurt Core Web Vitals, which then worsen one's search result rankings, we could see a shift in how publishers approach their ad strategies, with page speed factoring more into the equation.

This could lead to few potential scenarios:

  1. Fewer ad slots on the page
  2. Removal of the leaderboard banner at the top of the page (which affects these scores the most)
  3. Calling, say, four header bidding partners instead of eight to help with latency
  4. Eschewing programmatic ads and focusing more on a server-side, direct-sold approach, an approach increasingly made easy via ad serving APIs. Wired.com recently wrote an article about this, describing how a major publisher switched to a direct-sold approach and saw revenue increase 90%+.

Indeed, there are so many industry changes that are signaling a crisis for programmatic advertising in general, including GDPR/CCPA/other privacy laws, the deprecation of third-party cookies, mobile IDs going away, ad fraud concerns, and a growing awareness of ad tech middleman fees (about 50%).

No matter what, publishers should monitor their Core Web Vitals in the coming year. Even small downgrades to one’s search rankings can have a sizable impact on traffic and revenue. The update isn’t going live until 2021 – and developers can do their own testing with Chrome well before then.

Disclaimer: a similar article originally appeared on 7/31/2020 in AdExchanger



Chris Shuptrine

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