Web performance: ten metrics that shape whether a site works

In 2024, Google replaced FID with INP as a Core Web Vital — the biggest change to the metric set in five years. If your site still feels fine because of a score from an old version of PageSpeed Insights, it is time to recalibrate.

Core Web Vitals: the three metrics

These three metrics directly affect search rankings, and they are the performance dimensions users notice most:

1. LCP (Largest Contentful Paint): when the largest content element finishes rendering. Good is under 2.5 s.
2. INP (Interaction to Next Paint): the delay between an interaction and the next frame. Good is under 200 ms.
3. CLS (Cumulative Layout Shift): the total unexpected movement of page content. Good is under 0.1.

Four levers for LCP

LCP is usually the main first-screen image, the main headline, or the hero area. The optimization path:

• Preload critical assets. Use <link rel="preload"> to load the first-screen hero image and fonts earlier.
• Choose the right image format. Replace JPEG and PNG with WebP or AVIF; file size can drop by 30–60%.
• Serve responsive images. Use srcset so mobile devices download only the smaller image.
• Use a CDN and HTTP/2. Serve static assets close to the user; multiplexing reduces round-trip time.

INP: the new metric’s hidden killer

INP is much stricter than the old FID. FID measured only the first interaction; INP measures the 98th percentile of every interaction in a session. That means:

If your site has one complex modal, filter, or map component that a user opens occasionally, and it blocks the main thread for more than 200 ms, it can drag down the INP for the whole site.

The hidden traps in CLS

This is what a site with poor CLS feels like: you are about to click “Confirm”, an ad pushes the button down, and you click something else instead. Common causes:

• Images without explicit width and height, so space is only reserved after they load.
• Font loading that causes FOUT or FOIT, changing text size during the swap.
• Banners and cookie notices inserted dynamically.
• Embedded iframes without reserved height.

Seven more metrics worth watching

Beyond Core Web Vitals, these metrics also matter to the real user experience:

1. TTFB (Time to First Byte): server response speed, under 800 ms.
2. FCP (First Contentful Paint): first content rendered, under 1.8 s.
3. TBT (Total Blocking Time): total main-thread blocking time, under 200 ms.
4. Speed Index: how quickly the page fills in visually.
5. JS bundle size: under 200 KB gzipped per page is a healthy line.
6. Cache hit rate: above 90% on the CDN.
7. Error rate: fewer than one JavaScript error per 1,000 sessions.

The toolset

Our standard performance audit before delivery:

• Run Lighthouse for performance, accessibility, and SEO, with a target score above 90.
• Run WebPageTest from multiple locations and devices to inspect the waterfall on real networks.
• Use the Performance panel in Chrome DevTools to capture long main-thread tasks.
• After launch, use RUM (Real User Monitoring) to track real-user metrics at the 75th percentile continuously.

Performance is not finished when the site passes on launch day. Every new feature and every third-party script can push the numbers back down. Making performance monitoring part of the CI pipeline is the long-term fix.

Sources and caveats

The metric definitions and “good” thresholds above are Google’s published Core Web Vitals guidance, checked on 25 October 2026. Lab scores from Lighthouse or WebPageTest do not guarantee field results; performance varies with device, network, geography, and third-party scripts. The tool sequence is Lingyi’s delivery practice, not a formal standard. This article is a translation of the Chinese edition published on 22 October 2025.

1. Google web.dev, “Core Web Vitals,” accessed 25 October 2026. web.dev/articles/vitals.
2. Google web.dev, “Interaction to Next Paint (INP),” accessed 25 October 2026. web.dev/articles/inp.
3. Lingyi Information, “Web性能优化的10个关键指标与优化策略” (Ten key web performance metrics and optimization strategies), 22 October 2025. czlytech.com/news/web-performance-vitals.html.