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Can A CDN Cause Google CLS Problems? [2024] 💥

Can A CDN Cause Google CLS Problems?

The era of website speed and performance optimization has ushered in a greater focus on user experience, particularly with the introduction of Google's Core Web Vitals, which measures the quality of user experience. One of these vitals is the Cumulative Layout Shift (CLS). But can CDNs, which have been traditionally seen as a solution for speed improvement, inadvertently cause CLS problems?


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Understanding Google CLS

CLS is a metric that quantifies how often users experience unexpected layout shifts—a sudden jump of page content without user input. A low CLS means the page is stable; a high CLS can indicate a poor user experience.

Lab Data Vs. Real-World Data

It's essential to distinguish between lab and real-world data when discussing website performance. The lab data from tools like Lighthouse is primarily for debugging, offering indications of potential issues. However, the figures from lab data might not reflect real-world user experiences. Real-world data, on the other hand, is obtained using the CrUX tool. This tool captures genuine analytics from users who have usage statistics enabled in their browsers.

Notably, the mechanism capturing real-world data differs from the one obtaining lab data. Google Search Console solely utilizes real-world data to generate its page experience scores and to detect issues.

Priority of Data

When analyzing performance, the data's source hierarchy is paramount. If a URL receives ample real-world traffic from users with usage statistics enabled, CrUX generates a Field Data REPORT. If the URL lacks adequate data for this report, the system falls back on the origin summary report.

The Field Data Report is a comprehensive overview of metrics derived from real-world views. It considers metrics like First Contentful Paint (FCP), Largest Contentful Paint (LCP), CLS, and First Input Delay (FID), among others. A URL will be considered "good" if it hits a total score above 90% for at least 75% of real-world views. However, the URL must also be free of mobility, security, and ad experience issues.

When There's No Field Data Report

If a URL lacks a Field Data Report, it doesn't necessarily mean the absence of field data. Most likely, it indicates an insufficient amount of data to arrive at a reliable conclusion. In such cases, the URL will inherit scores from similar URLs that have the requisite Field Data Reports. Without any similar URL, the system leans on the origin summary.

The origin summary might be an aggregate of all views from all pages. Therefore, it's pivotal to maintain consistent quality across all pages to ensure a favorable origin summary.

Identifying and Addressing Issues

Even if a site has made significant improvements, old data can still negatively impact the origin summary. To remedy this, one must not only optimize pages but also generate more traffic. Increasing traffic influences averages, thereby bettering the overall score.

For individual pages, driving traffic to them can help generate a Field Data Report. As these reports are compiled, the overall health and performance of a website improve in Google's eyes.

Why Multiple PageSpeed Tests Yield Different Results

PageSpeed Insights, which uses the Lighthouse engine, might produce varying results due to multiple reasons:

  • Server response time fluctuations, impacted by traffic, load balance, the CDN's performance, and third-party resources.
  • The manner Lighthouse processes and interprets data, which can vary depending on small, unpredictable factors during analysis.
  • Differences in the virtual private server (VPS) environment where Lighthouse runs.

Real-world data introduces more variability since it accounts for diverse user devices, operating systems, browsers, and connection speeds.

The Impact of CDNs on CLS

Content Delivery Networks (CDNs) like Cloudflare are designed to distribute web content efficiently to users. However, CDNs can indirectly lead to CLS issues. For instance, if a CDN doesn't deliver assets consistently or if there's a delay in content rendering, it could trigger layout shifts.


Given the nuances of CLS and the potential for CDNs to introduce issues, it's risky to rely on them without thorough testing and monitoring. Once your CLS numbers dip below the 75% threshold, Google may penalize your site, leading to a significant decline in Search Engine Result Pages (SERP) performance. Specifically, one should be cautious about using free CDN services like Cloudflare, as they have been linked to CLS problems. Always prioritize user experience, regularly monitor site performance, and be discerning about the tools and services you implement.


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