• Initial Assessment and Benchmarking: Start by mentioning the importance of conducting an initial assessment of the application's current performance using tools like Lighthouse, PageSpeed Insights, or WebPageTest, Crux.
• benchmark key performance metrics such as load time, time to interactive, first contentful paint, and overall page size.
• core web vitals:'LCP, INP, CLS`, Loading, interactivity, stability
• elements affect LCP : image, block , video, background-image
• improve LCP : responsive images, compress, WebP, image cdn, server-push - push critical content by server, http2
• Improve CLS - width, height, skeleton pattern, reserved space for banners & dynamic content aspect-ratio for images, use low resolution placeholder images till it loads
• optimizing code, such as minification, compression, and tree shaking to reduce file sizes and improve load times.
• code splitting and bundling strategies using tools like Webpack to reduce initial load times
• Webpack, Rollup: Bundlers to optimize and bundle JavaScript, CSS, and assets for production.
• Code splitting: Technique to split the code into smaller chunks and load them on demand, improving initial load times.
• Service Workers: Utilized for caching assets and enabling offline functionality.
• Remove unused css purgeCSSPlugin
• Highlight the importance of optimizing images and other media assets using formats like WebP, lazy loading, and responsive image techniques.
• optimize images and media- WebP, lazy load, src-set, sizes, compress, loading='lazy', react-lazy-load-image-component, placeholder image, blur
• Image optimization tools: Tools like ImageMagick, Squoosh, or services like Cloudinary for optimizing images to reduce file size without compromising quality.
• Explain how optimizing the critical rendering path by minimizing render-blocking resources, leveraging browser caching
• inlining critical CSS and deferring non-critical CSS and JavaScript to prioritize above-the-fold content and reduce render-blocking.
• Virtualize large lists
• Performance hooks in React
• Server side rendering - Next.js
• Preload critical resources (e.g., above-the-fold content, essential scripts) and prefetch related resources (e.g., next page content, linked assets) to proactively load content and reduce perceived latency.

• Intermediate proxy servers or content delivery networks (CDNs) can cache resources on behalf of clients. These caches are closer to the user's location, reducing latency and server load.
• Server-Side Cache: Web servers can cache dynamic content or API responses to serve subsequent requests more quickly. Caching mechanisms like Memcached, Redis, or built-in server caching tools (e.g., NGINX caching) are used for this purpose.
• HTTP headers such as Cache-Control are used to control caching behavior. They specify directives like max-age, no-cache, no-store, public, private, and must-revalidate to instruct browsers and proxies on how to cache and handle cached content.
• Cached content needs to be invalidated or refreshed periodically to ensure that users receive updated information. Techniques like cache expiration (setting max-age or expires headers), cache busting (using unique query parameters or versioning), and manual cache purging are used for cache invalidation.
• optimizing network requests by reducing the number of HTTP requests, using HTTP/2 for multiplexing and server push, and implementing resource hints like preload and prefetch.
• server-side optimizations such as caching, CDN (Content Delivery Network) utilization, and Gzip compression to improve response times and reduce latency.
• Implement load balancing to distribute incoming traffic across multiple servers and improve application scalability. Use horizontal scaling (adding more servers) and vertical scaling (increasing server resources) to handle increased load.
• Enable compression (e.g., gzip, Brotli) for text-based assets like HTML, CSS, JavaScript, and JSON to reduce file sizes and improve transfer speeds

"You can't improve what you don't measure."

Tools:

• Google Lighthouse / Chrome DevTools Audits

• WebPageTest, GTmetrix

• Chrome Performance tab

• Core Web Vitals (CWV): Largest Contentful Paint (LCP), First Input Delay (FID), Cumulative Layout Shift (CLS)

• Real User Monitoring (RUM): New Relic, Datadog, Sentry, SpeedCurve

What to Look For:

• Slow Time to First Byte (TTFB)

• High LCP (>2.5s)

• Render-blocking resources

• Large JavaScript bundles

• Layout shifts or long input delays

• ✅ 2.1 Minify and Compress

Minify JS, CSS, and HTML (via Webpack, Terser, UglifyJS)

Use Gzip or Brotli compression on server

• ✅ 2.2 Use Efficient Image Formats

Convert to WebP, AVIF

Use responsive images with srcset

Use image CDNs (e.g., Cloudinary, Imgix)

• ✅ 2.3 Lazy Load Images and Components

Native loading="lazy" for

Lazy load components in frameworks (React.lazy, Vue async components)

• ✅ 3.1 Code Splitting

Split code by route or component

Use dynamic imports: import() in React, Vue, Angular

• ✅ 3.2 Tree Shaking

Remove dead code (ensure libraries support ES Modules)

Use sideEffects flag in package.json

• ✅ 3.3 Avoid Unused Dependencies

Audit with webpack-bundle-analyzer, source-map-explorer

Replace large libs (e.g., lodash) with smaller utilities (e.g., lodash-es, native functions)

• ✅ 3.4 Defer and Async Scripts

defer for JS scripts that don’t block rendering

async for independent scripts (analytics, ads)

• ✅ 4.1 Reduce Critical CSS

Inline critical CSS for above-the-fold content

Load the rest asynchronously

• ✅ 4.2 Eliminate Render-Blocking Resources

Move CSS to head (minified), JS to end of body (deferred)

• ✅ 4.3 Preload Key Resources

Fonts ()

Images

Hero content (e.g., banner image)

• ✅ 5.1 Browser Caching

Set far-future Cache-Control headers for static assets

Use cache busting (e.g., filename.[hash].js)

• ✅ 5.2 Service Workers

Implement caching strategies: stale-while-revalidate, cache-first

Use Workbox for PWA support

• ✅ 6.1 Skeleton Screens and Spinners

Show UI placeholders instead of blank screen

Improve First Contentful Paint (FCP) perception

• ✅ 6.2 Progressive Hydration (React, Next.js)

Only hydrate interactive parts as needed

• ✅ 6.3 Avoid Long Main Thread Tasks

Split heavy JS execution into chunks

Use requestIdleCallback or setTimeout to defer non-critical logic

• ✅ 7.1 Reduce HTTP Requests

Combine small assets (icons into SVG sprite)

Remove unnecessary polyfills

• ✅ 7.2 HTTP/2 or HTTP/3

Multiplex requests

Better header compression, faster TLS handshake

• ✅ 7.3 Debounce or Throttle API Calls

Avoid overloading the client or server

Prevent excessive re-renders

• ✅ 7.4 API Response Optimization

Compress API responses (Gzip, Brotli)

Avoid overfetching; use GraphQL or selective REST endpoints

• Step 8: Use a CDN and Global Edge Network ✅ Content Delivery Network (CDN)

Serve static assets close to users

Reduce latency for global users

React

Use React.memo, useMemo, useCallback effectively

Avoid prop drilling

Use Suspense for data fetching

Enable concurrent features (React 18+)

Vue

• Use computed properties instead of methods

• Avoid deep watching

• Use dynamic component loading

Next.js

• Enable Image Optimization

• Use Static Site Generation (SSG) where possible

• Use next/script for optimized script loading

Set up performance budgets

Monitor Core Web Vitals using RUM tools

Run automated Lighthouse audits in CI/CD pipelines

Use tools like Calibre, SpeedCurve, or GitHub Actions for performance regression testing

🔁 ### Continuous Loop: Audit → Optimize → Monitor → Repeat Treat performance like security — it's not one-and-done.

Profile and test across different devices and network conditions.

Balance between developer productivity, performance, and maintainability.

🧠 Bonus: Advanced Performance Techniques

HTTP caching headers (ETag, Last-Modified)

Web Workers for heavy computations

Priority Hints (importance="high")

Font Subsetting

Quicklink prefetching for next-page navigation