Why does JavaScript remain the most expensive resource for loading your pages?

What makes JavaScript so expensive compared to other resources?

Unlike pure HTML, which displays immediately once downloaded, JavaScript imposes a three-step processing pipeline: network download, parsing (conversion to machine bytecode), and then execution by the browser's JavaScript engine. Each step consumes CPU time and can potentially block the rendering of visible content.

The parsing alone can represent 10% to 30% of the total JS execution time on mobile, depending on the complexity of the code. Execution itself uses the browser's main thread — the very one that manages display and user interactions. Result: a 200 KB JS file can block rendering for several hundred milliseconds on an average Android device, while 200 KB of CSS or images only impact the network.

Why is Google emphasizing this point so much now?

Because JavaScript is ubiquitous in modern architectures: React, Vue, Angular frameworks, Next.js hydration, third-party widgets. The median weight of JS bundles on mobile has exploded, often exceeding 400 KB when compressed. Moreover, Googlebot crawls and renders millions of pages every day — every millisecond of JS parsing is costly in server resources and delays indexing.

The Core Web Vitals directly penalize sites that load too much blocking JS. The Largest Contentful Paint (LCP) and First Input Delay (FID, replaced by INP) mechanically suffer when the main thread is saturated by script execution. Therefore, Google encourages reducing dependency on critical JavaScript, not eliminating it — an essential nuance.

Is static HTML really always faster?

Yes, in absolute terms. A pure HTML document displays as soon as the browser receives the first bytes, without waiting for compilation or execution. It's mechanical: fewer steps = less latency. However, in practice, a modern site without JavaScript would lack interactivity, analytics tracking, and user personalization.

The goal is thus not a return to 2000s HTML but a hybrid architecture where the main content (text, headings, first paragraphs) is delivered in static HTML or server-side rendered (SSR), and where JavaScript loads only non-critical functionalities — lazy-loading, animations, widgets. The time to first byte (TTFB) and LCP benefit immediately from this approach.

• JavaScript imposes three costly steps: downloading, parsing, execution — while HTML/CSS only have one or two.
• JS parsing can consume 10% to 30% of the total time on mobile, especially with heavy frameworks.
• The Core Web Vitals penalize sites that block the main thread with too much JS.
• Static HTML or SSR remains the benchmark for instant display of critical content.
• The issue is not to eliminate JS, but to reduce its critical volume and defer the rest.

Is this statement consistent with field observations?

Absolutely. All Lighthouse, WebPageTest, or Chrome DevTools audits confirm that JS parsing and execution represent the main bottleneck on mobile. Poorly optimized React sites (bundles > 500 KB, blocking hydration) regularly show catastrophic FID or INP scores, even with a fast server and an efficient CDN.

But — and this is where it gets tricky — Google itself heavily uses JavaScript in its own services (Gmail, Maps, Search). This statement from Martin Splitt doesn't say “never use JS”; it says “be aware that JS is the most costly lever”. A crucial nuance: the idea is to budget JS like you budget crawl.

What nuances should be considered in 2025?

First, not all JavaScripts are created equal. A poorly written 50 KB script (nested loops, unnecessary re-renders) can block the main thread longer than an optimized 200 KB bundle with code splitting and lazy loading. The raw weight is only a proxy: the actual execution time is what matters.

Secondly, modern browsers (Chrome 110+, Safari 16+) have significantly improved their JS engines (V8, JavaScriptCore). Parsing is faster, and JIT is more aggressive. However, this improvement mainly benefits high-end devices: on a €150 Android, parsing remains two to three times slower than on an iPhone 14. If your audience is primarily mid-range mobile, this Google statement is even more critical.

[To be confirmed]: Google never specifies to what extent Googlebot itself is affected by JS parsing. We know the bot uses a version of Chrome, but with what CPU resources? How many pages does it render in parallel? These questions remain opaque, making it difficult to estimate the true SEO impact of a 300 KB bundle versus a 100 KB one.

In what cases does this rule not apply or is it less critical?

If your site is a pure Web application (like Notion, Figma, Trello), the experience relies entirely on JS. Your SEO job is then not to eliminate JavaScript but to ensure that Googlebot at least sees an HTML shell with title, meta description, and minimal indexable content. The rest can be client-side as long as the indexed landing page is coherent.

Another case: editorial sites with paywalls or strong personalization. If you display dynamic content based on the logged-in user, SSR or static HTML are not sufficient. Here, the challenge is to serve a static or pre-rendered version to Googlebot (via dynamic rendering or Server-Side Rendering) while keeping a rich client-side experience. Let's be honest: this dual architecture is complex and costly to maintain.

What should you do concretely to reduce the cost of JavaScript?

Immediate action: audit the weight and number of JS files loaded on your strategic pages. Open Chrome DevTools > Coverage, refresh the page, and identify scripts where less than 50% of the code is actually executed on the first render. These scripts are candidates for lazy loading or code splitting.

Next, urge your developers to adopt Server-Side Rendering (SSR) or static generation (SSG) via Next.js, Nuxt, Astro. The critical HTML content (titles, paragraphs, above-the-fold images) must be present in the source HTML, not injected afterwards by client-side JavaScript. Googlebot and users will see the content instantly, before even the JS is downloaded.

What mistakes should you absolutely avoid?

Never load heavy frameworks (React, Vue, Angular) to display a simple blog or a static showcase site. You would pay 200 to 400 KB of JS just to display text you could serve in pure HTML. Instead, use lightweight static generators (Hugo, 11ty, Jekyll) or a headless CMS with SSR.

Another classic trap: multiplying third-party scripts (Google Tag Manager, Hotjar, Intercom, Facebook Pixel, etc.). Each script weighs 30 to 100 KB and occupies the main thread. If you must keep them, load them deferred (defer) or better, async after the LCP. Never let a chat widget block the rendering of your product page.

How can you check if your site complies with this recommendation?

Run a Lighthouse audit (PageSpeed Insights or DevTools) and check the metrics “Reduce JavaScript execution time” and “Reduce unused JavaScript.” If you see alerts > 2 seconds of JS execution time, it’s red. Your goal: get below 1 second on mobile.

Then, test your pages on WebPageTest with a low-end mobile profile (Moto G4, slow 3G). Compare the Start Render and the Visually Complete with and without JS. If the gap exceeds 3 seconds, you have a structural problem. Finally, monitor your Core Web Vitals in Google Search Console: if more than 25% of your URLs fail on LCP or INP, JavaScript is likely the cause.

• Audit the Coverage in Chrome DevTools to identify unnecessary JS
• Adopt SSR or SSG to serve critical content in pure HTML
• Lazy-load non-critical scripts (chat, analytics, social embeds)
• Code-split the bundles to load only what is strictly necessary
• Defer or async third-party scripts to avoid blocking the LCP
• Test on low-end mobile (Moto G4, 3G) to validate real performance