What it is
Google measures page experience with three Core Web Vitals: Largest Contentful Paint (how soon the main content appears), Interaction to Next Paint (how quickly the page responds to a tap or click) and Cumulative Layout Shift (how much things jump around as it loads).
Lighthouse, the tool behind PageSpeed Insights, also gives an overall performance score from 0 to 100. It loads your page on a simulated mid-range phone over a throttled mobile connection, so it shows how the page feels to someone who isn't on fast office Wi-Fi.
Why it matters
Slow pages lose visitors
Every second spent looking at a blank or half-loaded screen is a chance for someone to give up and go back to the results. On a phone with a patchy signal that happens quickly, and each one is a lost enquiry or sale.
Speed is part of how Google ranks
Core Web Vitals are among the page experience signals Google uses in ranking. Good content still matters most, but between similar pages, the faster one has the edge.
Faster pages are easier to crawl
Search engines and AI crawlers have limited time for each site. Pages that respond quickly let them fetch more of your content.
Most causes are fixable
Slow pages usually come down to a few things: oversized images, render-blocking scripts and styles, heavy third-party tags, or a slow server. None of them needs a rebuild.
What Crawlable checks
- Lighthouse performance score
- Google Lighthouse's mobile performance score, the same number PageSpeed Insights shows. This is the Performance dial in your report.
- Largest Contentful Paint
- Good is 2.5 seconds or less; over 4 seconds is poor.
- Cumulative Layout Shift
- Good is 0.1 or less; over 0.25 is poor.
- Total Blocking Time
- How long the page is too busy running scripts to respond. On mobile, good is 200 ms or less; over 600 ms is poor.
- Real-visitor data
- Where Google has enough visits to your site, the Core Web Vitals your real visitors experienced, from the Chrome UX Report.
- Desktop
- A desktop run too, mentioned only when desktop is slow in its own right, since Google judges the mobile page first.
Common problems and how to fix them
A huge hero image
fetchpriority="high" so the browser fetches it first.Render-blocking scripts and styles
defer or async, and keep the CSS needed for the first screen small.Too many third-party tags
Content jumping as it loads
width and height attributes, or a CSS aspect-ratio, so space is reserved before they arrive. Avoid inserting banners above existing content.Questions
Why does my performance score change between runs?
Lighthouse is a lab test, and network and server timing vary a little each time, so scores move by a few points between runs. Look at the trend and at the individual metrics rather than a single number.
What's a good performance score?
Lighthouse treats 90 and above as good, 50 to 89 as needing improvement, and below 50 as poor. Real-visitor Core Web Vitals in the good range matter more than the lab score itself.
Does the Performance score affect my SEO score?
In Crawlable they're separate dials, but the SEO score does take Core Web Vitals into account, since Google does too.