What is a Performance Budget?
A performance budget is a limit for pages which you do not want to exceed. It is usually defined in terms of file size (e.g., 500KB total) or load time (e.g., LCP under 2.5s).
Why is LCP important?
Largest Contentful Paint (LCP) is a Core Web Vital metric that measures when the largest content element becomes visible. A good LCP score (under 2.5s) improves user experience and SEO rankings.
How is the budget calculated?
The calculator uses the formula: (Target Time × Network Speed / 8) × Efficiency Factor (0.85). The efficiency factor accounts for network latency and protocol overhead.
Does this budget include all assets?
Yes, the calculated budget represents the total transfer size (gzipped/compressed) of all resources (HTML, CSS, JS, Images, Fonts) needed to render the initial viewport.
What network speed should I target?
For a global audience, targeting 'Fast 3G' or '4G' is recommended to ensure your site is accessible to users with slower connections.
Why is the efficiency factor 0.85?
In real-world networks, you never get 100% of the theoretical bandwidth due to TCP slow start, handshakes, and headers. 0.85 is a conservative estimate for available throughput.
Can I exceed the budget?
You can, but it increases the risk of failing Core Web Vitals assessments, which can negatively impact your search ranking and user retention.
How do I reduce my page size?
Optimize images (WebP/AVIF), minify CSS/JS, use code splitting, lazy load below-the-fold content, and use efficient caching strategies.
Does this affect SEO?
Yes, Google uses Core Web Vitals as a ranking factor. Meeting performance budgets helps ensure you pass these metrics.
What is a good allocation for JavaScript?
JavaScript is the most expensive resource because it must be parsed and executed. Keep it as low as possible, ideally under 200KB compressed for the main bundle.