Website Performance Agency London

Core Web Vitals targets

Google publishes three thresholds for Core Web Vitals: Largest Contentful Paint under 2.5 seconds, Interaction to Next Paint under 200 milliseconds and Cumulative Layout Shift under 0.1 (per web.dev official documentation as updated for 2026). At Dedote we target stricter internal thresholds: LCP under 2.0 seconds, INP under 200 milliseconds and CLS under 0.05. Field data is measured via the Chrome User Experience Report (CrUX) because Google ranks on field data, not lab data. Akamai's 2017 retail study (still cited as a 2026 industry benchmark) showed that every 100 milliseconds of load delay reduces conversion by 7 percent.

Image pipelines

Images are the largest single performance liability on most marketing sites. We deliver AVIF first, WebP as fallback and original format as the last fallback, sized with srcset and sizes attributes for the actual rendered dimensions. The hero image uses fetchpriority="high" and link rel="preload" so it loads before the rest of the page. Cloudinary and Imgix 2026 data both report 60 to 80 percent file size savings on AVIF compared with JPEG at equivalent visual quality.

Bundle analysis and code-splitting

JavaScript bundles are the second largest performance liability after images. We run webpack-bundle-analyzer or vite-plugin-inspect on every build to surface what is shipping to the browser. Code-splitting at route level is standard. Dynamic imports for non-critical components. React 19 (released 2024) and Next 15 ship better default code-splitting than earlier versions, which simplifies the optimisation work. Bundle audits typically surface 30 to 50 percent reduction opportunities on legacy React projects we audit.

Font loading strategy

Custom fonts can block render for 500 to 2,000 milliseconds if loaded carelessly. We preload critical font files, use font-display: swap, host fonts locally where licensing allows and subset Google Fonts to only the characters the page actually uses. Google's official web.dev guidance recommends combining preload and font-display: swap as the standard 2026 pattern. The result is no flash of invisible text and no LCP hit from font loading.

CDN and edge caching

Edge caching reduces TTFB (Time to First Byte) by 100 to 500 milliseconds on UK traffic when origin sits outside London. We deploy on Vercel, Cloudflare, Netlify or Fly.io depending on the framework and data residency requirements. Static assets cached at edge with long max-age and immutable hashes. HTML cached with stale-while-revalidate. Cloudflare's 2026 performance report cites median TTFB reductions of 220 milliseconds for UK traffic served from London edge versus US origin.

Performance monitoring in production

Lab data lies. Field data tells the truth. We deploy real-user monitoring via Vercel Analytics, Cloudflare Web Analytics, SpeedCurve or a custom CrUX-based dashboard. Alerts fire when LCP, INP or CLS drift above target. Monthly reports show field data percentiles, not averages, because Google ranks on the 75th percentile of mobile data per official CrUX documentation.