What are Core Web Vitals and why do they matter for observability?

Core Web Vitals are a set of user-experience metrics defined by Google that measure loading performance, interactivity, and visual stability. They are directly included in Google's search ranking algorithm, making them both an observability concern and a business one.

The three current Core Web Vitals are:

LCP — Largest Contentful Paint: Measures when the largest image or text block in the viewport is rendered. Good: under 2.5 seconds. Poor: over 4 seconds. LCP is affected by slow server response times, render-blocking resources, and slow image loading.

INP — Interaction to Next Paint (replaced FID in 2024): Measures the latency of all user interactions (clicks, key presses) and reports the worst-case one. Good: under 200 ms. It replaces FID (First Input Delay) because FID only measured the first interaction, missing long-running JavaScript tasks mid-session.

CLS — Cumulative Layout Shift: Measures unexpected layout shifts — content jumping around while the page loads. Good: under 0.1. CLS is caused by images without dimensions, dynamically injected content above existing content, and web fonts causing FOUT (Flash of Unstyled Text).

From an observability perspective, Core Web Vitals are RUM metrics — they must be collected from real user browsers using the Web Vitals JavaScript library or a RUM agent. They complement server-side latency metrics because a server can respond in 50 ms while LCP is still 5 seconds due to client-side rendering bottlenecks.

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