Lighthouse and Core Web Vitals
Test any page's speed and quality with Google Lighthouse, understand every score, and fix what is slowing it down.
What it is
Lighthouse is Google's own page testing tool. IMVASA runs it for you on any page and returns four scores out of 100, the Core Web Vitals, other speed measurements, and the failing audits ranked by how much time each would save.
Where to find it: Lighthouse in the dashboard, or the lighthouse tool in your AI assistant. A test takes 20 to 60 seconds and costs about 4 credits.
Run a test
- Paste the address of one page.
- Choose mobile or desktop. Start with mobile: Google ranks pages based on their mobile version.
- Wait for the result. Run a second test if a score looks surprising, because network conditions vary between runs.
The four scores
| Score | What it measures | Good |
|---|---|---|
| Performance | How fast the page loads and becomes usable | 90 or more |
| Accessibility | Whether people using screen readers, keyboards or zoom can use the page | 90 or more |
| Best practices | Security and modern web standards, such as HTTPS and no browser errors | 90 or more |
| SEO | Basic search requirements, such as a title, meta description and crawlable links | 90 or more |
Scores from 50 to 89 need improvement. Under 50 is poor.
Core Web Vitals, explained
Core Web Vitals are Google's three measures of how a page feels. They are part of Google's page experience signals.
| Metric | Plain meaning | Good | Poor |
|---|---|---|---|
| LCP, Largest Contentful Paint | How long until the main content, usually the big image or headline, appears | 2.5 seconds or less | over 4 seconds |
| INP, Interaction to Next Paint | How quickly the page reacts when you tap or click | 200 milliseconds or less | over 500 milliseconds |
| CLS, Cumulative Layout Shift | How much things jump around while loading | 0.1 or less | over 0.25 |
Note: Lighthouse is a lab test: one simulated visit. INP needs real people interacting, so Lighthouse reports Total Blocking Time instead, which tracks the same problem. Google's ranking uses data from real Chrome users, which you can see in Google Search Console.
Other metrics you will see:
- First Contentful Paint: when anything first appears on screen.
- Speed Index: how quickly the visible page fills in.
- Total Blocking Time: how long the page was too busy running scripts to respond.
Fixing a slow LCP
- Compress the main image and serve it as WebP or AVIF at the size it is displayed.
- Do not lazy-load the main image at the top of the page.
- Preload the main image or font if it is discovered late.
- Speed up your server response with caching or a CDN.
- Remove or defer scripts that block the page from rendering.
Fixing layout shift (CLS)
- Give every image and video a
widthandheight. - Reserve space for ads, embeds and cookie banners before they load.
- Avoid inserting content above what the reader is already looking at.
- Use
font-display: swapand matching fallback fonts so text does not jump when web fonts arrive.
Fixing slow interaction (INP and Total Blocking Time)
- Remove scripts you do not need, especially old tracking tags and unused plugins.
- Load third-party widgets, such as chat and video, only when they are needed.
- Break long JavaScript tasks into smaller pieces so the page can respond in between.
Reading the failing audits
Below the scores, each failing audit shows an estimated saving in time or file size, plus the files responsible. Start with the biggest savings. If a third-party script tops the list, weigh its value against its cost: a chat widget that costs a second of load time on every page may not be worth it.
From your AI assistant
The lighthouse tool takes a url and an optional strategy of mobile or desktop. Try: "Run Lighthouse on brewlab.coffee on mobile and explain the three biggest problems as if I am not a developer."