The key website performance metrics are Largest Contentful Paint (LCP), Interaction to Next Paint (INP), Cumulative Layout Shift (CLS), First Contentful Paint (FCP), Time to First Byte (TTFB), Total Blocking Time (TBT), Speed Index, page weight, number of requests and page load time. Start with the three Core Web Vitals, and use the others to find the cause of a problem. Measure yours with our free website speed test.
Key takeaways
- The three Core Web Vitals (LCP, INP and CLS) are the metrics Google uses for page experience.
- Other metrics, such as TTFB and FCP, help you find where a delay happens.
- Lab metrics help you debug. Field metrics show what real visitors experienced.
- You do not need to track all ten every day. Focus on the Core Web Vitals and one or two diagnostics.
What are website performance metrics?
Website performance metrics are measurements of how fast a page loads, how quickly it responds and how stable it looks while loading. Some come from real visitors (field data) and some from a simulated test (lab data). Together they show how good the experience is and what to fix.
What are the 10 key website performance metrics?
| Metric | What it measures | Good value |
|---|---|---|
| 1. LCP | When the main content appears | 2.5 s or less |
| 2. INP | How quickly the page responds to input | 200 ms or less |
| 3. CLS | How much the layout jumps | 0.1 or less |
| 4. FCP | When the first content appears | 1.8 s or less |
| 5. TTFB | How fast the server responds | 0.8 s or less |
| 6. TBT | How long the page is blocked while loading (lab metric) | 200 ms or less |
| 7. Speed Index | How quickly the page fills in visually | 3.4 s or less on mobile |
| 8. Page weight | Total size of all files | No fixed target. Smaller is better |
| 9. Requests | Number of files the page loads | No fixed target. Fewer is better |
| 10. Load time | When the page finishes loading | No fixed target. Faster is better |
Thresholds for LCP, INP, CLS, FCP and TTFB come from Google’s guidance on web.dev. TBT and Speed Index values are the Lighthouse ranges for a good score on mobile.
Which metrics matter most for SEO?
The three Core Web Vitals matter most, because they are part of Google’s page experience signals. A page passes when LCP, INP and CLS meet the good thresholds for most real visits. See our guide to the Core Web Vitals test and how important page speed is for SEO.
What is the difference between lab and field metrics?
Lab metrics, such as TBT and Speed Index, come from a simulated test and are best for debugging. Field metrics, such as LCP, INP and CLS from real visitors, show the real experience. A page can look good in the lab and still fail in the field. See PageSpeed Insights accuracy.
How do you measure website performance metrics?
- Run a test with our free page speed checker or Google PageSpeed Insights.
- Check the Core Web Vitals first.
- Use TTFB, FCP and TBT to find the cause of a slow result.
- Check the Search Console report for real-user data across your site.
- Repeat after changes and compare the pattern.
To see how the score is built from these metrics, read what a good PageSpeed score is.
How do you improve these metrics?
- LCP and FCP: optimize images, use caching and reduce server response time. See how to fix LCP in WordPress.
- TTFB: improve hosting and caching. See what a good TTFB is.
- INP and TBT: reduce heavy JavaScript.
- CLS: set image dimensions and reserve space for banners.
Frequently asked questions
What are the most important website performance metrics?
LCP, INP and CLS, the three Core Web Vitals.
What is a good load time?
There is no single target. Aim for an LCP of 2.5 seconds or less on your key pages.
Is TBT a Core Web Vital?
No. TBT is a lab metric and a close stand-in for INP, which is measured in the field.
How often should I check these metrics?
Check after any major change, and review Search Console at least once a month.
Do I need to track all ten metrics?
No. Track the Core Web Vitals and add TTFB and FCP when you need to find a cause.
Conclusion
Know the three Core Web Vitals first, then use the other metrics to find causes. Test your key pages with our free website speed test and follow our guide on how to improve website speed.
