Why do your Core Web Vitals appear to deteriorate even as you optimize?

Why can degraded metrics be a positive indicator?

When you check your Core Web Vitals in Search Console and you notice a rise in LCP or CLS, your first instinct is to find out what has broken. Logical. Except that Martin Splitt raises a bias: if your site starts to rank in emerging countries or reaches rural audiences, these new visitors access via 2G or 3G networks. Their real performance drags your aggregated metrics down.

Specifically? You may have optimized your images, implemented a CDN, but your overall numbers are declining. Not because your site is slower, but because the composition of your audience has changed. Google measures the real user experience (CrUX), not that of a theoretical benchmark.

How can you differentiate a real regression from a composition effect?

The Search Console aggregates data from all your visitors without distinction. If 20% of your traffic now comes from India or Sub-Saharan Africa with 2G connections, your 75th percentiles will mathematically increase. This is not a technical regression, it's a demographic shift.

To make the distinction, segment your data. Use PageSpeed Insights with different throttling, or dive into BigQuery + CrUX to isolate performance by country, type of connection or device. If your historical users (desktop, 4G, France) maintain their performance but the new segments pull the overall down, you do not have a technical problem.

Does Google penalize a site whose metrics deteriorate for this reason?

This is the million-dollar question. Google communicates that Core Web Vitals are a ranking signal, but it has never specified how it weighs geographic segments or connection types in its evaluation. Theoretically, if your site performs well for the majority of your target visitors, you should not be penalized.

Let's be honest: there's a lack of transparency here. Google does not explicitly say whether it normalizes its thresholds by geographical area or applies the same global criteria. A French e-commerce site expanding internationally may see its CrUX scores deteriorate without any visible impact on French organic traffic - but it's hard to claim that this is systematic.

• Segment your data from Search Console and CrUX by country and device type to isolate the real performance of your historical audiences.
• Don’t panic over a global deterioration if your reach is expanding - first check the composition of your traffic.
• Optimizing for slow connections remains relevant: lazy loading, critical CSS, aggressive compression.
• Monitor business metrics (conversions, engagement) alongside Core Web Vitals to avoid focusing on vanity metrics.
• Document your hypotheses: if you launch a campaign in a new country, anticipate the impact on your aggregates and keep a history to contextualize variations.

Is this statement consistent with field observations?

Absolutely. We regularly observe websites that expand their geographic audience and see their Core Web Vitals Field Data deteriorate while nothing has changed on the front end. This is particularly visible for SaaS or media sites that launch localized versions in Southeast Asia or Latin America. 2G/3G connections are still predominant in some rural areas.

Where it gets complicated is that Google never explains how it weights these segments in its ranking algorithm. If 30% of your traffic now comes from India with catastrophic performance, does that impact your ranking in France? Probably not, but Google does not confirm this explicitly anywhere. [To be verified] on real cases with traffic/ranking correlations before and after geographic expansion.

What nuances should be added to this assertion?

First point: just because your metrics are deteriorating AND you are entering a new market does not mean one necessarily explains the other. You may very well have introduced a technical regression at the same time. Don’t hide behind the "new audience" argument without having verified with synthetic monitoring tools (WebPageTest, Lighthouse CI) that your intrinsic performance has not changed.

Second nuance: even if Google theoretically understands this phenomenon, there’s no guarantee it integrates it into its scoring. The CWV thresholds (2.5s for LCP, 100ms for FID, 0.1 for CLS) are global. If your site drops below these thresholds due to a new audience segment, you might be technically classified as "Poor" even if it's a false signal. The risk exists, although it is probably low for a site that maintains a good score with its historical audience.

In what cases does this rule not apply?

If your audience does not change geographically or demographically, a deterioration of your Core Web Vitals is a real warning signal. No new emerging market? Then you probably introduced blocking JavaScript, unoptimized images, or a third party that slows down your site. Don’t look for excuses.

Another case: if you're deliberately targeting emerging markets, you need to adapt your front-end. Google may overlook degraded global metrics, but your 2G users won’t forgive a site that takes 15 seconds to load. You lose on conversion, engagement, and likely local ranking. The argument "it’s normal, it’s 2G" doesn’t hold if you do nothing to compensate.

What actions should be taken to manage this phenomenon?

Your first action: segment your CrUX data. If you are using BigQuery, query the Core Web Vitals by country, type of connection (4G, 3G, 2G), and device. You can also cross-reference Search Console with Google Analytics 4 to identify which geographic areas pull your metrics down. Objective: to see if your historical visitors maintain their performance.

Then, optimize specifically for slow connections. Even if Google does not penalize you directly, your conversion rates will plummet if you serve 3 MB of JavaScript to a 2G user. Critical CSS inline, aggressive lazy loading, WebP images with ultra-compressed fallback, removal of non-essential third parties. The classics, but pushed to the limit.

What mistakes should be avoided when facing degraded metrics?

Error number one: panic and break everything. You see your Core Web Vitals turn red, you urgently refactor your front end, and you discover three months later that it was just an audience shift. Before touching the code, analyze the traffic composition.

Second error: ignore the problem on the grounds that "Google says it's normal." Google says a deterioration can be positive, not that it necessarily is. If you don't segment your data and assume everything is fine, you risk missing a real technical regression affecting everyone.

How can I check if my site remains performant despite audience expansion?

Implement synthetic monitoring with WebPageTest or Lighthouse CI. Set up regular tests from various geographical locations and connection types (3G Fast, 3G Slow, 2G). This gives you a baseline independent of the actual composition of your traffic. If your synthetic tests remain green but CrUX deteriorates, you know it’s a composition effect.

At the same time, track your business metrics by cohort. If your conversion rate in France remains stable but drops in India, your site is not suited for local connections. Conversely, if conversions follow everywhere, your aggregated Core Web Vitals may deteriorate without impacting your business or SEO.

• Segment Core Web Vitals by country and type of connection in BigQuery or a dedicated Analytics dashboard.
• Set up synthetic monitoring (WebPageTest, Lighthouse CI) from multiple geographical zones.
• Optimize specifically for slow connections: lazy loading, critical CSS, image compression, removal of non-essential JS.
• Track business metrics (conversions, bounce rate, time on page) alongside Core Web Vitals to contextualize variations.
• Document each geographical expansion or audience change to anticipate the impact on aggregated metrics.
• Never assume that a deterioration is "normal" without thorough investigation and data segmentation.