How fast is it on a phone, really?

The four scores

Performance is the one people quote, and the one that moves. It is a weighted blend of the lab metrics below, measured on a simulated slow 4G connection and a mid-range phone, which is why the mobile score is usually lower than desktop and is the one to fix. Accessibility, best practices and SEO are checklists: contrast, labels, HTTPS, console errors, meta tags. They should be at or near 100 on any professionally built site.

Core Web Vitals

• LCP (Largest Contentful Paint): when the main content is on screen. Good is 2.5 seconds or under. Usually driven by server response time, render-blocking scripts and oversized hero images.
• CLS (Cumulative Layout Shift): how much things jump around while loading. Good is 0.1 or under. Caused by images without dimensions, late-loading fonts, and banners inserted above content.
• TBT (Total Blocking Time) in the lab, and INP (Interaction to Next Paint) in real-user data: how long the page ignores taps. Driven by JavaScript, usually third-party tags and chat widgets.

Google uses the real-user versions of these, collected from Chrome over 28 days, as a ranking signal. They appear here when the page or site has enough traffic. Lab and field numbers differ; the field numbers are what your visitors experience.

Opportunities

Each item is an estimate of how much load time a change would save. Sort by that, not by count. One large hero image or one blocking script routinely accounts for more than every other item combined.

Common questions