If you've run your site through Google's PageSpeed Insights, you've seen the term Core Web Vitals. They're three measurements Google uses to describe how a page feels to real visitors. They're also one of many signals used in search ranking, so they're worth understanding even if you never touch code.
The three metrics
LCP: Largest Contentful Paint
What it measures: how long it takes for the biggest visible element (usually a hero image or a large heading) to appear.
Good: 2.5 seconds or less.
Why visitors care: this is the moment the page looks loaded. Until then, people are staring at a blank or half-built screen.
Common causes of a slow LCP:
- A huge, uncompressed hero image
- Slow server response time
- Fonts or scripts that block rendering
- Lazy loading applied to the hero image by mistake
INP: Interaction to Next Paint
What it measures: how quickly the page visually responds after someone taps, clicks, or types. INP replaced an older metric (First Input Delay) in 2024.
Good: 200 milliseconds or less.
Why visitors care: a menu that takes half a second to open feels broken. People tap again, and things get worse.
Common causes of a poor INP:
- Too much JavaScript running on the page
- Heavy third-party scripts such as chat widgets, trackers, and ad tags
- Large page builders that attach scripts to everything
CLS: Cumulative Layout Shift
What it measures: how much the layout jumps around while the page loads.
Good: 0.1 or less.
Why visitors care: you go to tap a button and an ad or image pushes it down at the last second. It's annoying at best and causes wrong clicks at worst.
Common causes of layout shift:
- Images without width and height attributes
- Ads, embeds, or banners that load in without reserved space
- Web fonts that swap in with very different sizes
Lab data vs field data
PageSpeed Insights shows two kinds of results:
- Field data comes from real Chrome users over the past 28 days. This is what Google uses for ranking signals, but it only appears if your site has enough traffic.
- Lab data is a single simulated test. It's useful for diagnosing problems, but scores can vary from run to run.
Don't panic over one lab score. Look for patterns, and fix the specific issues the report lists.
A realistic way to improve
- Start with the pages that bring in business. Your homepage and main service or product pages matter more than an old blog post.
- Fix the largest image on each page. Resize it, compress it, use a modern format, and make sure it isn't lazy loaded.
- Reserve space for everything. Set dimensions on images and embeds so the layout doesn't jump.
- Remove scripts you don't need. Every tag you delete helps INP.
- Re-test and repeat. Field data takes weeks to update, so give changes time.
Do perfect scores matter?
Not really. A score of 100 is nice, but the goal is a site that feels fast to real people on real phones. A clear, quick page that answers a visitor's question will always beat a perfect score on a page nobody wants to read.
