- Lighthouse performance scores reflect lab-tested page generation efficiency, weighted toward main-thread interaction blockers and visual assets.
- Total Blocking Time (TBT) evaluates main-thread execution delays; keeping it under 200 ms indicates low responsiveness risk.
- In lab environments, TBT acts as a proxy for interaction responsiveness.
- Largest Contentful Paint (LCP) benchmarks should remain under 2.5 seconds to ensure rapid visual delivery.
What is a website speed test?
A website speed test evaluates the efficiency of the connection path and rendering execution of a web page. Every page visit requires resolving hostnames, negotiating encryption, and executing HTML, CSS, and client-side JavaScript.
Understanding Core Web Vitals & Lab Proxies
Measures render time of the largest image or text block. Good targets are ≤ 2.5s.
Core Web VitalMeasures total main-thread blocking time between FCP and interactive state. Good targets are ≤ 200ms.
Lab Proxy for InteractionMeasures unexpected layout shifts while rendering. Good targets are ≤ 0.1.
Core Web VitalHow to improve performance
- Deploy an edge CDN: Cache static assets and HTML documents across a global Content Delivery Network to serve requests close to visitors.
- Optimize images and fonts: Serve modern formats (AVIF, WebP) and preload the primary LCP image asset.
- Minimize main thread blocking: Defer non-critical JavaScript to eliminate long tasks that inflate Total Blocking Time (TBT).
- Reserve layout dimensions: Always declare explicit aspect ratios on image and ad containers to eliminate CLS layout jumps.
Website speed FAQ
What is the Lighthouse Performance Score?
The Lighthouse Performance score is an aggregate 0 to 100 benchmark calculated from weighted log-normal curves: First Contentful Paint (10%), Speed Index (10%), Largest Contentful Paint (25%), Total Blocking Time (30%), and Cumulative Layout Shift (25%).
Why do lab data and field data differ?
Lab data comes from a single run on a controlled, emulated environment (Headless Chromium on Google servers). Field data (CrUX) aggregates 28-day telemetry from diverse real-world users on varying devices, networks, and battery states.
What is Total Blocking Time (TBT)?
Total Blocking Time measures the total duration between First Contentful Paint (FCP) and Time to Interactive (TTI) where the browser's main thread was blocked by tasks exceeding 50 ms. In lab runs, TBT acts as a proxy for interaction responsiveness.
What is Speed Index?
Speed Index measures how quickly contents are visually populated during page load using frame-by-frame visual progression analysis.
Builds and maintains PC Tester diagnostic utilities. Methodologies align with Google Lighthouse and W3C Web Vitals criteria.