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home / developersection / interviews / how does preloading or deferring css/js affect core web vitals?
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Anubhav Sharma
Anubhav Sharma

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The Anubhav portal was launched in March 2015 at the behest of the Hon'ble Prime Minister for retiring government officials to leave a record of their experiences while in Govt service .

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1 Answers

Anubhav Sharma
Anubhav Sharma 06 Apr 2025
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Preloading or deferring CSS and JavaScript has a big impact on Core Web Vitals — especially LCP, FID, and even CLS indirectly. When done right, these techniques reduce load time, unblock rendering, and free up the main thread for user interactions.

Here's how each technique affects Core Web Vitals:

1. Preloading

Preloading tells the browser "Hey, I’ll need this resource soon, please fetch it early."

Affects:

  • LCP (Improves it)
  • CLS (Preloading fonts helps prevent layout shifts)
  • INP/FID (by making key assets ready faster)

What to Preload:

  • Fonts (to prevent FOUT/FOUT)
  • LCP image
  • Above-the-fold background images
  • Critical scripts (like above-the-fold UI logic)

Example:

<link rel="preload" href="/fonts/Inter.woff2" as="font" type="font/woff2" crossorigin="anonymous" />
<link rel="preload" href="/images/hero.jpg" as="image" />

2. Deferring JavaScript

Deferring scripts avoids blocking the browser from rendering the page.

Affects:

  • FID / INP (less blocking on first interaction)
  • LCP (allows content to render earlier)

Use defer or async:

<script src="/main.js" defer></script> <!-- runs after HTML parsed -->
<script src="/analytics.js" async></script> <!-- runs as soon as ready -->

Tip: Use defer for scripts that depend on DOM structure. Use async for independent scripts like analytics.

3. Deferring Non-Critical CSS

Too much CSS can delay rendering — especially if it’s render-blocking.

Affects:

  • LCP (critical CSS allows faster paint)
  • If not careful, may cause CLS if content flashes unstyled and shifts after style loads.

How to Optimize CSS:

  • Extract critical CSS (e.g., using tools like Critical or Penthouse).
  • Load the rest async:
<link rel="stylesheet" href="styles.css" media="print" onload="this.media='all';" />
<noscript><link rel="stylesheet" href="styles.css"></noscript>

Common Mistakes

Mistake Impact
Lazy-loading LCP image Hurts LCP
Blocking main thread with large JS Hurts FID/INP
Not preloading fonts Causes layout shift (CLS)
Inlining too much JS/CSS Slows TTFB, can bloat HTML

Tools for Diagnosis

  • Lighthouse / PageSpeed Insights: Shows unused JS/CSS and render-blocking resources.
  • Chrome DevTools → Performance tab: See what blocks rendering and user input.
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