---
title: "How does preloading or deferring CSS/JS affect Core Web Vitals?"  
description: "How does preloading or deferring CSS/JS affect Core Web Vitals?"  
author: "Anubhav Sharma"  
published: 2025-04-06  
updated: 2025-04-06  
canonical: https://www.mindstick.com/interview/34030/how-does-preloading-or-deferring-css-js-affect-core-web-vitals  
category: "web development"  
tags: ["web development"]  
reading_time: 4 minutes  

---

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

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:

```html
<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`:

```html
<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:

```html
<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.

## Answers

### Answer by Anubhav Sharma

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:

```html
<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`:

```html
<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:

```html
<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.


---

Original Source: https://www.mindstick.com/interview/34030/how-does-preloading-or-deferring-css-js-affect-core-web-vitals

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