Is JavaScript truly the worst enemy of Core Web Vitals?

What practical steps can be taken to reduce JavaScript impact? <\/h3>

First lever: audit critical JavaScript<\/strong> using the Coverage Tool in Chrome DevTools. Identify the code actually executed at loading versus dead or deferrable code. On most sites, 60 to 80% of the initial JS is unnecessary for First Paint.<\/p>

Second lever: code splitting<\/strong> and aggressive lazy loading. Load only the bundle necessary for the current page, defer everything else. Webpack, Rollup, and Vite offer native configs for this. On an e-commerce site, the JS of the checkout funnel has no business on the homepage.<\/p>

Third lever: migrate to SSR or SSG<\/strong> when possible. Next.js, Nuxt, SvelteKit allow for rendering HTML server-side and progressively hydrating. The gain on LCP is immediate, often 1 to 2 seconds on 3G mobile.<\/p>

What mistakes should be absolutely avoided? <\/h3>

Never load heavy JavaScript libraries<\/strong> (Lodash, Moment.js) to use a single function. Prefer targeted imports or lightweight alternatives (date-fns, day.js). A bundle that goes from 400 KB to 150 KB can halve the TBT.<\/p>

Avoid render-blocking scripts<\/strong> in the without defer or async attributes. Each synchronous script blocks HTML parsing and delays First Paint. Loading jQuery at the top of the page remains a common headache on legacy sites.<\/p>

How can I check if my site meets Google's recommendations? <\/h3>

Use PageSpeed Insights<\/strong> and Lighthouse to measure TBT (Total Blocking Time), which directly correlates with FID. A TBT under 200 ms is acceptable; above 600 ms you're in the red zone.<\/p>

Enable CPU throttling 4x<\/strong> in Chrome DevTools to simulate a low-end mobile device. If your site becomes unusable, it's a sign that JavaScript is the main bottleneck. WebPageTest with a Moto G4 profile on slow 3G provides a realistic view of the user experience.<\/p>

• Audit critical JS with the Coverage Tool and remove dead code<\/li>
• Implement code splitting and lazy loading on all non-critical bundles<\/li>
• Migrate to SSR/SSG for high-traffic pages (homepage, categories, product sheets)<\/li>
• Replace heavy libraries with light alternatives or vanilla code<\/li>
• Defer or async all third-party scripts (analytics, tag managers, A/B testing)<\/li>
• Measure TBT and FID in real conditions with PageSpeed Insights and CrUX<\/li><\/ul>
  The impact of JavaScript on Core Web Vitals is not a fatality. With a methodical optimization strategy<\/strong>, most sites can halve their TBT and gain 10 to 20 points on their mobile score. Gains are measurable within weeks. For complex architectures (SPA React/Vue, custom e-commerce platforms), these optimizations require deep web performance expertise. If your team lacks resources or technical skills, hiring a specialized SEO agency<\/strong> in Core Web Vitals can significantly accelerate results and avoid costly mistakes in time and positioning.<\/div>