Website Performance Checklist
Crushing Core Web Vitals: Images, Caching, CDNs, and critical rendering paths.
Performance is no longer just a technical nice-to-have; it's a fundamental feature of your product. Slow websites suffer from astronomical bounce rates, abysmal conversion metrics, and severe penalties in search engine rankings. If your site doesn't load instantly, your users will leave before they even see what you have to offer.
This checklist provides a deep dive into the performance optimization phase of our Ultimate Website Launch Checklist. It details actionable, developer-centric strategies to minimize payload size, reduce server response times, and achieve passing grades for Google's Core Web Vitals.
Related Technical Guides:
1. Mastering Core Web Vitals
Google explicitly uses Core Web Vitals as ranking signals. You must measure and optimize these three metrics to ensure a fast, stable user experience.
Largest Contentful Paint (LCP)
LCP measures loading performance. It marks the time it takes for the largest text block or image within the viewport to become visible. Aim for an LCP of 2.5 seconds or less. To improve LCP, optimize your server response time (TTFB), aggressively compress images, defer non-critical CSS/JS, and implement server-side caching or a CDN.
Interaction to Next Paint (INP)
INP replaced FID (First Input Delay) as the metric for interactivity. It assesses a page's overall responsiveness to user interactions (clicks, taps, keyboard input) by observing the latency of all interactions that occur throughout the lifespan of a user's visit. Aim for an INP of 200 milliseconds or less. To improve INP, minimize main-thread work by breaking up long tasks, deferring non-essential JavaScript execution, and optimizing complex DOM manipulations.
Cumulative Layout Shift (CLS)
CLS measures visual stability. It quantifies how much the page layout shifts unexpectedly during the loading process (e.g., text jumping down when a slow-loading ad or image finally renders). Aim for a CLS score of 0.1 or less. The most effective way to eliminate CLS is to explicitly include `width` and `height` attributes on all `<img>` and `<iframe>` elements, ensuring the browser allocates the correct space before the asset downloads.
2. Asset Optimization: Images, CSS, and JS
The fastest request is the one that never happens. When a request is necessary, ensure the payload is as small as possible.
Image Compression and Modern Formats
Images often account for the vast majority of a page's total weight. Never serve raw images straight from a camera or design tool. Use automated tools or build steps to compress images without noticeable quality loss. Convert legacy formats (JPEG, PNG) to modern, highly efficient formats like WebP or AVIF, which offer superior compression ratios.
Native Lazy Loading
Implement lazy loading for all images and iframes located "below the fold" (not immediately visible upon page load). Use the native HTML attribute `loading="lazy"`. This defers the download of these assets until the user scrolls near them, dramatically decreasing the initial payload and speeding up the LCP of critical above-the-fold content.
Minification and Bundling
Ensure your production build process minifies all HTML, CSS, and JavaScript. Minification strips unnecessary whitespace, removes comments, and shortens variable names, resulting in significantly smaller file sizes. While HTTP/2 reduces the need for aggressive bundling, logically grouping assets can still reduce parsing time and network overhead.
3. Caching Strategies and Delivery Networks
If you must send data over the wire, make sure it travels the shortest possible distance and doesn't need to be regenerated unnecessarily.
Content Delivery Network (CDN) Architecture
A CDN is non-negotiable for a modern web application. CDNs distribute your static assets across a global network of edge servers. When a user requests your site, the CDN routes them to the closest physical server. This drastically reduces latency (Time to First Byte - TTFB) and offloads traffic from your origin server.
Aggressive Browser Caching (`Cache-Control`)
Configure HTTP response headers to instruct the user's browser to cache static assets locally. For versioned, immutable assets (like CSS/JS files with unique hashes in their filenames: `app-v2.js`), use aggressive caching: `Cache-Control: public, max-age=31536000, immutable`. This ensures repeat visitors load these assets instantly from their local disk rather than making a network request.
Early Hints and Preloading
Optimize the critical rendering path by providing hints to the browser. Use `<link rel="preload">` for high-priority resources discovered late in the parsing process (like web fonts or hero images). Use `<link rel="preconnect">` to establish early connections to important third-party origins (like an analytics provider or a separate API domain) before the browser actually needs to make a request.
4. Server and Infrastructure Optimization
Frontend optimizations can only do so much if your backend is slow.
Enable Gzip or Brotli Compression
Ensure your web server (Nginx, Apache, or your hosting platform) is configured to compress text-based responses (HTML, CSS, JS, JSON, SVG) before sending them to the client. Brotli generally offers better compression ratios than Gzip, but Gzip enjoys universal browser support. Configure your server to negotiate the best algorithm.
Database Optimization and Indexing
If your application relies on a database, slow queries will kill your TTFB. Ensure database tables are properly indexed, optimize complex queries, and utilize database connection pooling. Consider implementing an in-memory caching layer (like Redis or Memcached) to store the results of frequent, expensive database queries.
Static Site Generation (SSG)
Whenever possible, generate pages at build time rather than on demand. Static Site Generators (like Astro, Next.js, or Hugo) pre-render HTML, meaning the server only has to serve static files. This eliminates database queries and server-side rendering overhead during a user's request, resulting in near-instantaneous load times and massive scalability.