Speed Index measures how fast the above-the-fold area of a page fills in visually. Lighthouse or WebPageTest records the load as video frames, compares each frame with the final one and computes the average time at which the viewport looked complete. Lower is better.
Why it matters
Single-point metrics such as FCP and LCP miss the shape of loading. Two pages can both finish at 3 seconds, but one may show 90 percent of its content at 1 second while the other stays blank until 2.9 seconds. Speed Index rewards the first pattern.
Lighthouse scores it against thresholds of roughly 3.4 seconds or less as good for mobile, and over 5.8 seconds as poor.
Example
Page A: 50% visual at 0.8s, 90% at 1.5s, 100% at 3.0s
Page B: 0% until 2.5s, then 100% at 3.0s
Same load end, but Page A has a much lower Speed Index.Common mistakes
- Treating it as a field metric. It cannot be collected from real users.
- Expecting it to improve when only below-the-fold images are optimized.
- Using skeleton screens or spinners and believing they help; they may count as visual progress or not, depending on what is painted.
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