How does preloading or deferring CSS/JS affect Core Web Vitals?

Question

Anubhav Sharma · Answer

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.

interview

How does preloading or deferring CSS/JS affect Core Web Vitals?

Can you answer this question?

1 Answers

1. Preloading

Affects:

What to Preload:

Example:

2. Deferring JavaScript

Affects:

Use defer or async:

3. Deferring Non-Critical CSS

Affects:

How to Optimize CSS:

Common Mistakes

Tools for Diagnosis

Use `defer` or `async`: