Page speed and conversion

Studies by Google, Akamai and Amazon have consistently demonstrated the direct relationship between speed and conversion. Amazon estimated that every 100ms of delay cost them 1% of sales. Walmart observed that every second of speed improvement increased conversion by 2%.

In ecommerce, the impact is especially critical in mobile commerce, where conversion rates are already lower than on desktop. A website that takes 5 seconds to load on mobile loses more than 50% of its visitors before they see a single line of content.

Core Web Vitals are three Google-defined metrics that measure loading experience: LCP (Largest Contentful Paint) measures when the main content renders, INP (Interaction to Next Paint) measures responsiveness to interactions, and CLS (Cumulative Layout Shift) measures visual stability.

Google’s performance thresholds are: LCP under 2.5 seconds (good), INP under 200ms (good), and CLS under 0.1 (good). Meeting these thresholds improves both SEO and user experience, which are two sides of the same coin.

The highest-impact optimisations tend to be the most basic: compressing and resizing images, minifying CSS/JS, enabling gzip/brotli compression and configuring cache. These actions alone can cut loading time in half on many websites.

For further improvements, advanced techniques include: lazy loading images and videos below the fold, preloading critical resources (fonts, above-the-fold CSS), removing unused JavaScript (tree shaking), and CDN to reduce geographical latency.

On mobile, speed depends on factors you cannot control: the user’s connection quality (3G, 4G, unstable WiFi) and their device’s processing power (not everyone has the latest iPhone). Optimising for mobile means optimising for the reasonable worst case.

Google recommends testing your site on a simulated 4G connection and a mid-range device. If your website performs well under those conditions, it will perform well for most of your mobile audience. Chrome DevTools lets you simulate these exact conditions.

Google PageSpeed Insights provides field data (real users from the Chrome UX Report) and lab data (simulated). Field data is what Google uses for ranking and is the most representative of real-world experience. WebPageTest offers more detailed analysis with resource waterfalls and filmstrips.

For continuous monitoring, tools like SpeedCurve or Calibre alert you when performance degrades. Integrating speed checks into your CI/CD pipeline (with Lighthouse CI) prevents performance regressions before they reach production.

Investing in speed has a direct return in conversions and an indirect return in SEO. To calculate ROI, estimate how many additional conversions a specific speed improvement would generate based on industry data.

If your site receives 50,000 visits/month, converts at 2% and each conversion is worth €100, a 1-second LCP improvement that increases conversion by 10% would represent 1,000 additional conversions per year × €100 = €100,000. The cost of optimisations rarely approaches those figures.