How do you improve Core Web Vitals?

Improving Core Web Vitals is a common topic in frontend performance optimization, and it often comes up in interviews for web development roles. These metrics—Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS)—are Google's way of quantifying user experience around loading speed, interactivity, and visual stability. Understanding how to improve them requires both a solid grasp of browser behavior and practical experience tuning real-world applications.

Let me walk you through what Core Web Vitals are, why they matter, and how to improve them with practical examples and considerations you’ll likely face in production.

What Are Core Web Vitals?

Core Web Vitals are a set of user-centric performance metrics that Google uses to evaluate the quality of a webpage’s user experience. They focus on three main areas:

• Largest Contentful Paint (LCP): Measures how long it takes for the largest visible content element (like an image or heading) to render on the screen.
• First Input Delay (FID): Measures the time from when a user first interacts with your page (click, tap, keyboard input) to when the browser actually begins processing that interaction.
• Cumulative Layout Shift (CLS): Measures the visual stability of a page by tracking unexpected layout shifts during the page lifecycle.

These metrics are critical because they directly impact how users perceive your site’s speed and responsiveness. Slow or janky sites frustrate users and lead to higher bounce rates.

How to Improve Largest Contentful Paint (LCP)

LCP is all about how quickly the main content appears on the screen. Here are some practical ways to improve it:

1. Optimize Critical Rendering Path

The browser needs to download, parse, and execute HTML, CSS, and JavaScript before it can render the page. Anything blocking this process delays LCP.

• Minimize CSS blocking: Inline critical CSS for above-the-fold content and defer non-critical CSS using media attributes or rel="preload".
• Defer or async JavaScript: Scripts that aren’t needed immediately should be loaded asynchronously or deferred to prevent blocking rendering.

2. Optimize Images and Media

Images often contribute to the largest content element on the page, so optimizing them is key:

• Use modern formats like WebP or AVIF for better compression.
• Serve appropriately sized images based on device screen size using srcset and sizes.
• Lazy-load offscreen images to prioritize above-the-fold content.
• Use Content Delivery Networks (CDNs) to reduce latency.

3. Use Server-Side Rendering (SSR) or Static Site Generation (SSG)

Rendering HTML on the server means the browser gets fully formed content faster, improving LCP. Frameworks like Next.js or Gatsby make this easier.

4. Reduce Server Response Times

Slow backend responses delay when the browser can start rendering. Use caching, database optimizations, and efficient APIs to speed up server responses.

How to Improve First Input Delay (FID)

FID measures how quickly your page responds to user input. It’s mostly about JavaScript execution and main thread availability.

1. Break Up Long Tasks

JavaScript that runs for hundreds of milliseconds blocks the main thread, making the page unresponsive. Use code-splitting, web workers, and requestIdleCallback to break up heavy work.

2. Optimize JavaScript Payloads

• Remove unused code with tree shaking.
• Minify and compress scripts.
• Load only essential scripts initially; defer or lazy-load the rest.

3. Avoid Heavy Third-Party Scripts

Third-party scripts (analytics, ads, widgets) can introduce significant delays. Audit and remove unnecessary ones or load them asynchronously.

How to Improve Cumulative Layout Shift (CLS)

CLS tracks unexpected layout shifts that annoy users, like buttons or images jumping around while the page loads.

1. Reserve Space for Images and Ads

Always specify width and height attributes or use CSS aspect-ratio boxes to reserve space before content loads. This prevents layout shifts when images or ads load asynchronously.

2. Avoid Injecting Content Above Existing Content

Inserting banners, pop-ups, or dynamic content above visible content causes shifts. Instead, reserve space or insert content below the fold.

3. Use Font Loading Strategies

Flash of unstyled text (FOUT) or flash of invisible text (FOIT) can cause layout shifts. Use font-display: swap in CSS to minimize this.

Common Mistakes Developers Make

• Ignoring mobile performance: Many developers optimize for desktop but forget mobile devices have slower CPUs and networks.
• Overusing third-party scripts: Adding multiple analytics or ad scripts without auditing their impact can tank FID.
• Not measuring real user metrics: Synthetic tests are useful, but real user monitoring (RUM) reveals actual performance issues.
• Not prioritizing visible content: Loading large images or heavy scripts before above-the-fold content delays LCP.
• Forgetting to reserve space for dynamic content: This causes CLS and frustrates users.

Performance Considerations

Improving Core Web Vitals is a balancing act. For example, deferring JavaScript improves FID but might delay interactive features. Similarly, aggressive image optimization can reduce quality, impacting user perception.

Use tools like Lighthouse, WebPageTest, and Chrome DevTools to identify bottlenecks. Also, monitor real user metrics with tools like Google Analytics or Web Vitals JS library to understand how your users experience the site.

Security Considerations

While Core Web Vitals focus on performance, security can indirectly affect them. For instance, serving content over HTTPS is essential for HTTP/2, which improves loading speed. Also, Content Security Policy (CSP) headers can prevent malicious scripts that might degrade performance or cause layout shifts.

Interview Tips

• Explain Core Web Vitals clearly and why they matter for user experience and SEO.
• Use concrete examples from your experience, such as how you optimized images or refactored JavaScript.
• Discuss trade-offs, like balancing image quality vs. size or deferring scripts vs. interactivity.
• Mention tools you use for measuring and monitoring performance.
• Be ready to talk about how you’ve handled CLS issues, which are often overlooked.

Practical Production Scenario

In one project, we had a React-based e-commerce site with poor LCP and high CLS scores. The largest content was a hero image and product title. We:

• Implemented server-side rendering with Next.js to send fully rendered HTML.
• Used next/image to serve optimized images with lazy loading for offscreen assets.
• Added explicit width and height to images and reserved space for dynamic banners to fix CLS.
• Split JavaScript bundles and deferred non-critical scripts to improve FID.
• Removed a heavy third-party chat widget that was blocking the main thread.

After these changes, LCP dropped from 4.5s to 1.8s, FID improved from 300ms to 50ms, and CLS was nearly zero. This resulted in better user engagement and improved search rankings.

Comparison Table: Core Web Vitals Optimization Techniques

Metric	Optimization Technique	Benefits	Trade-offs
LCP	Server-Side Rendering (SSR)	Faster initial content display	More complex build and deployment
LCP	Image Optimization (WebP, lazy loading)	Reduced load time and bandwidth	Potential quality loss if over-compressed
FID	Code Splitting & Deferring JS	Improved interactivity and responsiveness	Initial complexity in build setup
FID	Reducing Third-Party Scripts	Less main thread blocking	Loss of some analytics or features
CLS	Reserve Space for Images/Ads	Visual stability, better UX	Requires upfront layout planning
CLS	Font Loading Strategies (font-display: swap)	Prevents layout shifts from font swapping	May cause brief flash of unstyled text