In web development, web performance is how quickly a website loads, becomes usable and reacts to input. MDN also counts how smooth scrolling and animations feel. The term here does not cover marketing performance or server hardware.
What is measured?
MDN distinguishes objective measurements, such as load time and frames per second, from perceived performance, which is how fast the site feels to a person. Measurements come from two sources: lab tests under fixed conditions (for example Lighthouse) and field data from real visitors (real user monitoring, CrUX).
Google’s Core Web Vitals cover three aspects:
| Metric | Aspect | Google’s target |
|---|---|---|
| Largest Contentful Paint (LCP) | loading | within 2.5 seconds |
| Interaction to Next Paint (INP) | responsiveness | under 200 milliseconds |
| Cumulative Layout Shift (CLS) | visual stability | under 0.1 |
What is the link to Google Search?
Google’s documentation recommends good Core Web Vitals and says they align with what its core ranking systems seek to reward, together with other page experience aspects. Relevant content remains a separate matter.
What influences it?
web.dev’s Learn Performance course covers the critical rendering path, CSS and JavaScript delivery, images and video, code splitting, lazy loading, caching and web fonts.
Example
A product page has a 2 MB hero image that loads late, so field LCP is 3.8 seconds. After resizing the image and loading it earlier, the value falls below 2.5 seconds.
Sources
- Web performance – MDN Web Docsdeveloper.mozilla.org
- Understanding Core Web Vitals and Google search results – Google Search Centraldevelopers.google.com
- Learn Performance – web.devweb.dev
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