Should you really trust the page speed score for optimizing your SEO?

Why does Google question its own speed scores?

Mueller's statement comes at a time when many SEOs obsessively focus on the overall score displayed by PageSpeed Insights or Lighthouse. This number — between 0 and 100 — is supposed to synthesize a page's performance. However, this synthesis oversimplifies a much more nuanced reality.

Google's tools aggregate several performance metrics (First Contentful Paint, Speed Index, Largest Contentful Paint, Time to Interactive, Total Blocking Time, Cumulative Layout Shift) into a single score. Each metric has a different weight, and the calculation changes regularly. As a result, two pages with the same score can offer radically different user experiences.

What makes these scores misleading in practice?

The major issue is that the score is calculated in a testing environment — often on a fast server, with a stable connection. The real conditions of your users (3G mobile, low CPU, active browser extensions) are never accurately reflected.

Additionally, the score does not distinguish between cosmetic optimizations and structural improvements. You can gain 15 points by deferring three non-critical scripts without addressing the real bottleneck that slows your LCP by 2 seconds. Google encourages you not to stop at the number but to explore the detailed diagnostics.

Which metrics should be monitored instead?

Mueller emphasizes specific insights: loading time of the main content (LCP), visual stability (CLS), responsiveness to interactions (FID/INP). These metrics comprise the Core Web Vitals, used as a ranking factor since 2021.

In practical terms, if your LCP is at 4.2 seconds while the recommended threshold is 2.5s, you have a precise target. If your CLS spikes to 0.3 because of a poorly implemented carousel, you know where to intervene. The overall score, however, will never tell you where or how to act — it simply judges you.

• The overall score is a weighted average that conceals crucial details for UX and ranking.
• The Core Web Vitals (LCP, CLS, FID/INP) are the metrics Google truly uses as a ranking signal.
• Lab testing conditions (PageSpeed Insights) do not reflect real-world data (Chrome UX Report) — prioritize real data.
• Optimizing for the score can lead to counterproductive trade-offs (aggressive lazy-loading, removal of useful features).
• Detailed tools (Lighthouse in trace mode, WebPageTest, Search Console) offer concrete action points that the score alone never provides.

Is this statement consistent with observed practices on the ground?

Absolutely. For years, we have seen sites with a PageSpeed score of 50-60 ranking in the top 3, while competitors at 95+ stagnate on page 2. The score is not a direct ranking factor — it's the underlying metrics that matter, and even so, their weight remains moderate compared to content relevance or domain authority.

Let’s be honest: many agencies sell “PageSpeed optimization” services centered around the green number. The client sees their score jump from 65 to 92 and is happy, but the organic traffic doesn’t budge because the real issue (a catastrophic LCP on mobile 4G) was never addressed. Mueller points this out.

What nuances should be added to this official position?

First nuance: not all tools are created equal. PageSpeed Insights combines lab data (Lighthouse) and real-world data (CrUX). If you don’t have enough traffic to feed CrUX, you will only see lab data — and yes, the score becomes even less reliable. [To be checked]: Google has never specified the exact traffic threshold to appear in CrUX, but it is estimated that several thousand monthly visits on Chrome are needed.

Second nuance: the score remains a useful alert indicator. A site with a score of 20/100 likely has serious structural issues (undersized server, uncompressed images, massive blocking JS). But between 60 and 95, variations are often noise — and chasing those last points is rarely profitable.

In what cases does this rule not fully apply?

If you operate in an ultra-competitive sector (fashion e-commerce, travel, finance) where all major players are at 95+ on desktop and mobile, neglecting the score could cost you dearly. Not because Google penalizes you directly, but because your competitors have optimized their real Core Web Vitals — and the high score is the indirect consequence.

Another edge case: AMP sites or those with aggressive CDN. The score can be artificially inflated by caching, while the user experience on the uncached version remains poor. Again, the number deceives — but in the opposite direction.

What should you do concretely to effectively optimize speed?

Stop looking at the overall score first. Open PageSpeed Insights, scroll down to the detailed Core Web Vitals, and identify the metrics exceeding the “Good” thresholds. If your LCP is at 3.8s, your number one priority is to reduce that LCP — not to gain 5 score points by optimizing Time to Interactive.

Then, cross-reference lab data with real-world data. In Google Search Console, under “Core Web Vitals,” you will see which real URLs are problematic for your users. A site may have a lab score of 85 but a catastrophic CrUX if the majority of traffic comes from low-end mobiles in Southeast Asia. It’s this real traffic that counts for ranking.

What mistakes should be avoided when optimizing speed?

Don’t fall into the trap of excessive lazy-loading. Some sites defer loading above-the-fold content to improve FCP but ruin their LCP. Google measures when the largest visible element appears — if it’s a lazy-loaded image, you’re losing time.

Another classic pitfall: optimizing only the homepage. Product pages, categories, and blog articles often have very different performance profiles. A good score on the homepage means nothing if your product listings, which generate 80% of revenue, are at 40/100 on mobile. Audit a representative sample of your critical templates.

How can I check that my site complies with Google’s recommendations?

Use Search Console as your source of truth. The CrUX data displayed there is what Google actually uses for ranking. If all your critical URLs are in the green (LCP < 2.5s, CLS < 0.1, FID < 100ms), you’re in the clear — regardless of whether PageSpeed Insights gives you a score of 67 or 94.

Supplement with WebPageTest by simulating realistic user profiles (3G mobile, CPU throttling). You will see metrics that Lighthouse doesn’t show, like Start Render or Visually Complete. And install Google’s Web Vitals extension to monitor your own browsing — nothing beats testing in real conditions.

• Identify Core Web Vitals metrics outside thresholds (LCP, CLS, FID/INP) through PageSpeed Insights and Search Console
• Cross-reference lab data (Lighthouse) with real-world data (CrUX) to prioritize optimizations
• Audit critical templates (product listings, landing pages, articles) and not just the homepage
• Avoid aggressive lazy-loading on above-the-fold content that penalizes LCP
• Monitor the evolution of Core Web Vitals over time with Search Console and third-party tools (WebPageTest, SpeedCurve)
• Test in real conditions (low-end mobile, 3G, CPU throttling) to validate that optimizations hold in production