Web Perf: the web performance guide (2026)

What is Web Performance Optimization (WPO)?

WPO covers the techniques that speed up downloading, rendering and responsiveness of pages. Speed influences search ranking (Core Web Vitals), conversion and accessibility. Measure with PageSpeed Insights (CrUX field data and Lighthouse lab test) and the Search Console page experience report.

Where to start, by role

• Web developer: INP, CLS, defer and async, lazy loading, Preload.
• DevOps: server configuration, HTTP caching, Brotli compression, HTTP/3, CDN.
• SEO: Core Web Vitals, robots.txt, sitemap and llms.txt, Speculation Rules.
• Webmaster / WordPress: image optimization, Caching, plus WordPress .htaccess tuning (in French).

Common causes of slowness

• overloaded or distant hosting → Server;
• heavy images or wrong formats → Images;
• outdated CMS, theme or plugins, technical debt;
• oversized DOM, too many div/span instead of semantic tags;
• uncontrolled third-party scripts (ads, analytics, chat) → INP;
• no caching or CDN → Caching.

The “good” Core Web Vitals thresholds are detailed in the dedicated guide; the cheat sheet sums up every method.

In this section

• Core Web VitalsCore Web Vitals explained: LCP ≤ 2.5 s, INP ≤ 200 ms, CLS ≤ 0.1 at the 75th percentile. Field data, tools and an action plan for each metric.
• OptimizationWeb performance optimization: AVIF/WebP images, HTTP caching, Brotli compression, HTTP/3, robots.txt, sitemap.xml and llms.txt.
• Advanced techniquesAdvanced loading techniques: preload, Speculation Rules (prefetch/prerender), native lazy loading, defer and async scripts.
• Rendering enginesCompare the Blink (Chrome, Edge, Brave), Gecko (Firefox) and WebKit (Safari) rendering engines: caveats and resources.
• Cheat sheetCheat sheet of web optimization methods (Core Web Vitals, loading, crawl, server, accessibility) with benefits and examples.