Web Design & Performance

Page Speed and Conversions: How Load Time Affects Your Revenue

By Harrison Hill· Founder & Chief Strategist
11 min read

Speed is a business metric, not a technical one. Every 100ms of additional load time costs real revenue: higher bounce rates, lower conversion rates, and worse search rankings. Google has explicitly stated that page speed is a ranking factor, and Core Web Vitals are now part of the page experience signal.

Yet most businesses treat page speed as a "nice to have" or a developer's problem. The data tells a different story. The gap between a 1.5-second load and a 4-second load is the difference between industry-leading conversion rates and losing half your potential customers.

At iNDEXHILL, web design and technical SEO are inseparable. This guide covers the real-world impact of speed on revenue and the practical steps to fix it.

The Speed-Revenue Correlation

The relationship between page speed and conversions isn't theoretical. Data from large-scale studies consistently shows that faster pages convert significantly better.

Page Speed vs Conversion Rate

Real-world correlation from speed optimisation project (ecommerce)

  • speed
  • conversion

Reducing page load time from 4.2s to 1.8s over 12 weeks doubled conversion rates from 1.8% to 3.8%. The steepest gains came between weeks 4-8, where load time dropped below 3 seconds — the threshold where most users abandon mobile sessions.

View full data table
PeriodLoad Time (s)Conversion (%)
Week 14.2s1.8%
Week 23.8s2%
Week 43.1s2.4%
Week 62.5s2.9%
Week 82.1s3.3%
Week 121.8s3.8%

The chart above tracks a real ecommerce site through a 12-week speed optimisation project. As load time dropped from 4.2s to 1.8s, conversion rates more than doubled. This pattern is consistent across industries and business models.

Key Statistics

  • 1-second delay — Reduces conversions by 7% and page views by 11%
  • 3-second load — 53% of mobile visitors leave before the page finishes loading
  • 5-second load — Bounce probability increases by 90% compared to 1-second load
  • Sub-2-second load — Average ecommerce conversion rates are 2-3x higher than sites loading in 4+ seconds

The Mobile Gap

Mobile connections are typically slower than desktop. If your site loads in 2 seconds on a desktop fibre connection, it might take 5-6 seconds on a 4G mobile connection. Since mobile traffic now exceeds 60% for most businesses, mobile speed is the metric that matters most.

Core Web Vitals: What Google Measures

Google's Core Web Vitals are three specific metrics that measure user experience. Since 2021, they've been a confirmed ranking signal.

The Three Vitals

  • LCP (Largest Contentful Paint) — How quickly the main content loads. Target: under 2.5 seconds. This is usually the hero image, headline, or main content block
  • INP (Interaction to Next Paint) — How responsive the page is to user input. Target: under 200ms. This replaced FID in 2024 and measures all interactions, not just the first one
  • CLS (Cumulative Layout Shift) — How much the page layout shifts during loading. Target: under 0.1. Layout shift happens when elements load and push other content around

How to Measure

  • PageSpeed Insights — Google's free tool showing both lab and field data
  • Chrome DevTools — Performance tab for detailed waterfall analysis
  • Google Search Console — Core Web Vitals report showing site-wide field data
  • Web Vitals Chrome Extension — Real-time CWV measurement while browsing your site

Image Optimisation: The Biggest Quick Win

Images account for 50-80% of total page weight on most websites. Optimising images is typically the single highest-impact speed improvement you can make.

Format Selection

  • WebP — 25-35% smaller than JPEG at equivalent quality. Supported by all modern browsers. This should be your default
  • AVIF — 20% smaller than WebP but slower to encode. Use for hero images where quality matters most
  • SVG — For logos, icons, and illustrations. Infinitely scalable, tiny file size
  • JPEG — Fallback only. Use progressive JPEG for perceived speed improvement

Sizing and Loading

  • Responsive images — Use srcset and sizes attributes to serve appropriate image dimensions for each device
  • Lazy loading — Add loading="lazy" to all images below the fold
  • Width and height attributes — Always specify dimensions to prevent CLS
  • Image CDN — Use Cloudflare Images, Imgix, or similar for automatic format conversion and resizing

Hero Image Strategy

The hero image is usually the LCP element. Prioritise it: preload via <link rel="preload">, don't lazy-load it, and consider using a low-quality placeholder (LQIP) that transitions to the full image.

Code and Resource Optimisation

After images, JavaScript and CSS are the next biggest contributors to slow pages.

JavaScript Optimisation

  • Code splitting — Load only the JavaScript needed for the current page, not the entire application
  • Defer non-critical scripts — Analytics, chat widgets, and social embeds should load after the page is interactive
  • Tree shaking — Remove unused code from bundles during build
  • Third-party audit — Each third-party script (analytics, pixels, live chat) adds weight. Audit them quarterly and remove anything not providing clear value

CSS Optimisation

  • Critical CSS — Inline the CSS needed for above-fold content directly in the HTML <head>
  • Remove unused CSS — Tools like PurgeCSS can reduce CSS file sizes by 80-95%
  • Avoid @import — Use <link> tags instead, as @import blocks parallel downloads

Server and Infrastructure

  • CDN — Serve all static assets from a global CDN (Cloudflare, AWS CloudFront, Fastly)
  • HTTP/2 or HTTP/3 — Enable multiplexing for parallel resource loading
  • Compression — Enable Brotli compression (20-30% smaller than gzip)
  • Caching headers — Set long cache durations for static assets with cache-busting via filename hashing

Speed and SEO: The Ranking Impact

Page speed affects SEO rankings both directly (as a confirmed ranking signal) and indirectly (through user behaviour metrics).

Direct Ranking Impact

  • Core Web Vitals — Part of Google's page experience ranking signal since 2021
  • Crawl efficiency — Faster sites allow Googlebot to crawl more pages in its allocated crawl budget
  • Mobile-first indexing — Google uses the mobile version of your site for indexing. Mobile speed matters most

Indirect Ranking Impact

  • Bounce rate — Slow pages increase bounce rates, which correlates with lower rankings
  • Dwell time — Users who wait for slow pages spend less time engaging with content
  • Pages per session — Slow sites discourage exploration, reducing internal page views

For competitive SEO queries, speed can be the tiebreaker between otherwise equal pages. It's rarely the primary ranking factor, but it's the easiest one to optimise.

Speed Optimisation Checklist

Use this checklist to systematically improve your site's speed. Work through items in priority order: images first, then code, then infrastructure.

Quick Wins (This Week)

  1. Convert images to WebP format
  2. Add lazy loading to below-fold images
  3. Add width and height attributes to all images
  4. Defer third-party scripts (analytics, chat, social embeds)
  5. Enable Brotli compression on your server or CDN

Medium Effort (This Month)

  1. Implement responsive images with srcset
  2. Audit and remove unused CSS
  3. Set up a CDN if not already using one
  4. Implement code splitting for JavaScript
  5. Inline critical CSS for above-fold content

Strategic (This Quarter)

  1. Move to a faster hosting platform if TTFB exceeds 400ms
  2. Audit all third-party scripts and remove non-essential ones
  3. Implement preloading for critical resources
  4. Consider a headless architecture for content-heavy sites
  5. Set up continuous monitoring with SpeedCurve or similar

How we do this at iNDEXHILL

Our Web Design services are built around this exact framework, designed for businesses that need predictable growth.

See how we applied this approach in our client case studies.

Frequently Asked Questions

Aim for a PageSpeed Insights score of 90+ on mobile and desktop. More importantly, target real-world metrics: LCP under 2.5s, INP under 200ms, and CLS under 0.1. Lab scores and field data often differ, so prioritise the field data from your actual users.

Yes. Core Web Vitals have been a confirmed ranking signal since 2021. Speed is rarely the primary ranking factor (content relevance and authority matter more), but for competitive queries where other factors are equal, faster pages will rank higher.

Studies consistently show 7-10% conversion improvement per second of reduced load time. For a site generating £100,000/month in revenue, improving load time from 4s to 2s could mean an additional £14,000-20,000 per month.

Mobile. Google uses mobile-first indexing, and most sites get 50-70% of traffic from mobile devices. Mobile connections are also inherently slower than desktop, so optimisations here have greater impact on user experience.

Want help implementing this?

If you're looking to scale organic growth, we offer a free SEO audit to identify quick wins and growth opportunities.

Request a free SEO audit

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