Core Web Vitals, minus the jargon
Core Web Vitals are three measurements of how a page feels to a real visitor: how fast the main content appears, how quickly the page responds when you tap something, and whether things jump around while it loads. Google uses them as a ranking signal, but the better reason to care is that slow, jumpy pages lose customers.
LCP: how fast the main content appears
Largest Contentful Paint is the time until the biggest image or block of text in the viewport has rendered. Good is 2.5 seconds or less.
What usually fixes it:
- Serve the hero image at the size it’s displayed, in a modern format like WebP or AVIF.
- Don’t lazy-load the hero image. Lazy loading is for images further down the page.
- Preload the hero image or font if the browser discovers it late.
- Cut server response time with page caching. A slow first byte delays everything after it.
INP: how quickly the page responds
Interaction to Next Paint measures the delay between a tap, click or key press and the screen updating. Good is 200 milliseconds or less. It replaced First Input Delay as a Core Web Vital in March 2024.
What usually fixes it:
- Ship less JavaScript. Every plugin that adds a script adds work for the browser.
- Load third-party scripts (chat widgets, trackers, ads) after the page is interactive.
- Break long-running scripts into smaller pieces so the browser can respond in between.
CLS: whether things jump around
Cumulative Layout Shift scores how much visible content moves unexpectedly while the page loads. Good is 0.1 or less.
What usually fixes it:
- Give every image and video a width and height, so the browser reserves the space.
- Reserve space for embeds, ads and cookie banners instead of pushing content down.
- Choose fallback fonts that are close in size to your web fonts.
Lab data versus field data
Tools like Lighthouse run a test on one simulated device. They’re good for debugging, but Google’s assessment uses field data: measurements from real Chrome visitors, judged at the 75th percentile. A page passes when at least three out of four visits hit the “good” threshold for all three metrics.
Search Console’s Core Web Vitals report shows the field data for your own site, and that’s the number worth watching. It’s also where our website speed work starts, and how we judge whether it worked.