The Complete Website Performance Checklist for 2026
A fast, well-structured website is the foundation of every other digital investment. Use this checklist to benchmark yours against what the best-performing sites actually do.
Why performance is a business problem, not just a technical one
Every second of delay in page load time costs roughly 7% of conversions. Every visual shift after the page loads erodes trust. Every render-blocking script delays the moment your visitor can act. Website performance is not a concern for your developer to worry about in isolation — it is directly tied to revenue, bounce rate, SEO rankings, and how your brand is perceived on first contact.
This checklist covers the areas that matter most. It's designed to be used by founders, marketing leads, and product teams — not just engineers.
Core Web Vitals
Google's Core Web Vitals are the clearest performance benchmark for 2026. The three signals that matter most are Largest Contentful Paint (LCP), which measures how quickly the main content loads; Cumulative Layout Shift (CLS), which measures visual stability; and Interaction to Next Paint (INP), which replaced First Input Delay and measures responsiveness.
- LCP target: Under 2.5 seconds. Your hero image or headline should load fast.
- CLS target: Under 0.1. Reserve space for images before they load to prevent layout jumps.
- INP target: Under 200ms. User interactions should feel instant.
Run your site through Google PageSpeed Insights and treat anything below 90 on mobile as a conversion problem.
Image optimisation checklist
- All images are in WebP or AVIF format (not JPEG or PNG)
- Images have explicit width and height attributes to prevent layout shifts
- Above-the-fold images use
loading="eager"andfetchpriority="high" - Below-the-fold images use
loading="lazy" - Images have responsive
sizesattributes so the browser downloads the right size - No image is larger than it needs to be at its largest display size
JavaScript and CSS checklist
- No render-blocking scripts in the
<head>withoutdeferorasync - Third-party scripts (analytics, chat, heatmaps) are loaded after the page is interactive
- Unused CSS is purged — especially if you use a utility framework
- Critical CSS is inlined so the above-the-fold view doesn't require a separate stylesheet request
- JavaScript bundles are code-split so only what's needed on each page is loaded
Font and typography checklist
- Fonts use
font-display: swapto prevent invisible text during load - Font files are preloaded for the most critical weights
- Multiple font families are loaded in a single request, not separate stylesheets
- Variable fonts are used where possible (one file covers all weights)
Infrastructure checklist
- The site is served from a CDN so assets are geographically close to visitors
- HTTP/2 or HTTP/3 is enabled for multiplexed asset delivery
- GZIP or Brotli compression is enabled on the server
- Static assets have long cache TTLs with cache-busting on deploy
- The server has a Time to First Byte (TTFB) under 600ms
Mobile performance checklist
- The site passes Google's mobile usability test with zero errors
- Tap targets are at least 48×48px with adequate spacing
- No horizontal scrolling on any viewport width
- The page is tested on a real low-end device, not just a browser emulator
- Forms are easy to complete with a mobile keyboard (correct input types)
What to do with this checklist
Run through each section honestly and mark what passes and what doesn't. The items with the highest conversion impact are Core Web Vitals on mobile, image optimisation, and script loading. Fix those first. The rest follows naturally as part of a well-built site.
If you're starting from scratch or rebuilding, a purpose-built website will handle most of this checklist by default — because performance is engineered in from day one rather than retrofitted later.
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