This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung →
Checking Speed Index: how quickly the screen fills up
Speed Index describes how quickly the visible area fills with content overall. Unlike LCP or FCP, it doesn't name a single point in time — it assesses the whole course of rendering.
- This value is especially useful for pages that build up gradually: two pages can have the same LCP, but one feels empty the whole time while the other shows content from the start.
- Because it's calculated from a sequence of frames, it only exists in the lab — it isn't collected in real visitors' browsers.
FAQ
Is Speed Index a Core Web Vital?
No. It's one of the lab metrics and feeds into the performance score, but it isn't one of the three core values.
When should you pay attention to it?
When the individual values look good, but the page still feels sluggish. The course of events then often shows that the visible area stays empty for a long time.