Google CLS & High Impact Ad Formats

Updated: Mar 30



Adnami has recently been asked by several of our publishing partners for advice on how they best navigate around Google’s upcoming introduction of its Core Web Vitals, which will be added to the Page Experience Ranking Factor from May 2021.

The factor Page Experience Signals is very technical and only one of over 200 ranking factors that are taken into account for measuring website performance. In addition to good content and the factors from the Quality Rater Guidelines, the Core Web Vitals are an important influence and should certainly be taken into consideration. However, in the overall picture of optimizing websites, they play a subordinate role.

Source (google)


Google Core Web Vitals is made up of three elements that webmasters will have to keep in mind in the future: loading, interactivity and visual stability.

  • Loading - Largest Contentful Paint (LCP) - How quickly does the page load?

  • Interactivity - First Input Delay (FIC) - How quickly can a user interact with the site?

  • Visual stability - Cumulative Layout Shift (CLS). Visual stability on a website.

How is Your Overall Core Web Vitals Ranking Score Calculated?

Your performance score is made up by the different search signals under the core web vitals framework, and the signals have different weights. Loading and Interactivity have by far the biggest impact on the overall performance score, while Visual Stability (CLS) only accounts for 5% of the overall performance score. This means the best way to optimize for your overall core web vital performance is to work with your performance around loading and interactivity.


You can find your site’s Core Web Vitals data in the “enhancements” section of your Google Search Console account.

If you are looking for general advice on how to optimise for Google’s core web vitals, we recommend you take a closer look at the Core Web Vital website.

What is CLS?

When a website loads, layout shifts can happen pushing elements down the page, for instance, while the next element is loading above. When content shifts as a page loads and the user is already interacting with the site, this can have undesirable effects from a user perspective. This is what should in general be prevented.

How is CLS calculated? CLS is calculated using this formula: Impact fraction * distance fraction = your CLS score. Read more here for more technical information about how CLS is calculated.

What is considered a good and a bad score?

Scores between 0-0.1 are considered good. Scores between 0.1-0.25 are considered to need improvement. Scores between 0.25 and above is considered poor

It is important to note, the overall significance of a CLS score (good or bad) varies significantly from website to website. For some sites, organic search traffic accounts for 1-2% of their traffic, while for others it accounts for 95%. Subsequently, the more your website traffic relies on traffic coming from search, the more important google’s core web vitals performance will be to you.

How much do High Impact Formats Affect the CLS Measure and Overall Core Web Vitals Ranking?

We tested Adnami formats on four different publisher websites. These sites had a variety of different performance scores before we tested, the results we considered are the averages. We tested the performance of each format on each site 20 times.

We have broken it out specifically to CLS, as well as the overall performance of the sites. Below you will see how the Adnami formats affect both of those metrics.

CLS & Overall Results - overall core web vitals score is not affected by impact formats, but CLS is in isolation is

In the “CLS Scores”chart below you see the average CLS scores on Baseline (blue) compared to those when loading in Topscroll (green), Midscroll (red) and Skins (yellow).

CLS Scores


When you look at CLS in isolation, the units all deliver a negative CLS score, where the Topscroll format has the worst impact on CLS.

Overall Core Web Vitals Ranking Scores

While the effect High Impact formats have on your CLS score seems worrying at first glance, they clearly cannot be looked at without the full picture being considered.



Looking at the overall performance score, the four sites measured came out with an improved overall performance when High Impact formats were initiated on the sites, compared to the tested websites base performance score.


This is likely because other standard ads, which are loaded onto the tested pages in the base environment, were slower to load compared to the tested impact ads. Load time represents a much bigger factor, than visual stability, in the calculation of a website’s overall core web vital performance.


Moreover, while impact formats seem to improve the overall Core Web Vitals Ranking, impact formats also deliver 5-8 timers higher yield for publishers using the formats, and up to 10 times higher ad effectiveness compared to standard display ads.


What can publishers do to optimize towards CLS without compromising ad revenue from running high Impact formats?


  • Reserve the ad space. To fix the layout shift, you can reserve the space that is later filled out with the ad. Here you can find some inspiration on how to do that.

  • You can control frequency by ad unit to ensure your users don’t get exposed to big and impactful ads more than once or twice per session. This will decrease the overall CLS score and keep your users happy, whilst still monetizing this premium ad format.

  • Make sure to respect the page position of the user. The default setting in Adnami’s topscroll certifications is to respect the position of the user when our formats load. However, sometimes we see publishers overwrite this setting and create a layout shift to make sure the ad is loading in screen. E.g. if a user has scrolled down, say 10% of the browser window’s height, before a topscroll unit has loaded, then the topscroll ad is loading above the fold. This will have a negative effect on viewability and ad performance, but we recommend optimizing for fast page-, and ad loading times, rather than forcing a layout shift for the user.

  • Make sure you price accordingly for topscroll to account for the potential effect of a lower core web vitals score.

  • You can look to replace topscroll inventory with alternative high impact formats such as Skins and Midscrollers, where the impact on overall web vital performance is close to none. To improve viewability on Midscrollers, you can consider to move them up on your page, so they sit “on the fold”.


What can Adnami do to assist publishers' in achieving great CLS scores and Core Web Vitals Ranking?


At Adnami we are always working to develop innovative ad formats that deliver great user experiences for consumers, increased yield and revenue for publishers and better returns for advertisers. In that order. During Q2 we will introduce new formats and performance features that will further increase the positive effects from running high impact advertising. The first announcements will be made in April already.


In summary, although high impact formats will affect your CLS score, little has changed on how it affects your overall performance, and actually in a lot of cases overall performance is improved. Any media company's consideration to reduce high impact ad inventory should be offset against the economic loss in yield and revenue.


We, as a company, will work with you to help ensure any negative impact is minimal and give you advice on how to optimize your website for a better overall web vitals performance.


The aim of this “help article” is not to account for everything related to Google’s Core Web Vitals, so for more general information about the framework we recommend reading more about it online, where there are lots of helpful articles.


Please don’t hesitate to get in touch for questions or thoughts on this topic.



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