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Performance
What does a website's Speed Index measure?
The Speed Index measures how fast a page's visible area fills with content — Google rates up to 3.4 seconds as good, above 5.8 seconds as poor. Unlike values that capture a single moment, the Speed Index looks at the entire build progression. It therefore tells apart a page that becomes visible piece by piece from one that stays blank for a long time and then suddenly appears complete.
What counts is the progression of the page build, not just a single moment
Two pages can be equally fast once fully loaded and still feel completely different. Page A shows the heading after one second, the text after two seconds, everything after four. Page B stays blank for four seconds and then jumps onto the screen fully formed. Visitors perceive page A as noticeably faster, even though both finish at the same time — because they get something to see and read early on. The Speed Index captures exactly that: it tracks, at short intervals, what percentage of the visible area is already filled, and condenses that progression into a single number. The earlier a lot becomes visible, the better the value.
Speed Index is a lab measurement with fixed thresholds
The Speed Index comes from the lab: a test run loads the page under fixed conditions — a throttled connection, a defined device — and records how the screen builds up. The value is calculated from the individual frames. There's no field data from real visitors for this metric, since recording the screen is only possible in a controlled test run. The Google thresholds: up to 3.4 seconds good, up to 5.8 seconds needs improvement, above that poor. The audit service collects the value via Google's PageSpeed measurement, separately for mobile and desktop, and reports every value above the threshold as a finding with the measured value.
A poor Speed Index points to the loading order
If the Speed Index is poor even though individual values like the first content are fine, the loading order is usually the issue: the page first loads things that aren't visible — scripts, content far down the page, third-party tools — and puts the upper, visible area last. The fix is prioritisation: whatever the visitor sees first has to come first. Images below the visible area can load later, scripts for chat or analytics don't need to run before the content. The audit service's report gives your agency the measured value and the context of which other metrics are flagged at the same time.
- Watch when opening your homepage whether the screen fills piece by piece, or stays blank for a long time and then appears complete all at once.
- Ask your agency to load images below the visible area only when scrolled to, so the upper area gets priority.
- Have scripts for chat widgets, analytics, and ads load after the visible content instead of before it.
- Compare the Speed Index with the first-content value (FCP) in the audit report: if only the Speed Index is poor, it's the loading order, not the server.
- Check the value for mobile and desktop separately, since the throttled mobile measurement exposes ordering problems much earlier.
FAQ
What is the Speed Index?
The Speed Index is a metric from the lab measurement that assesses the entire progression of the visible page build. The test run records how the screen fills up and condenses that into a number in seconds. The earlier large parts of the visible area are in place, the better the value — a page that stays blank for a long time scores poorly.
What Speed Index value is good?
Google rates a Speed Index up to 3.4 seconds as good, up to 5.8 seconds as needs improvement, and above that as poor. The audit service measures against exactly these thresholds, separately for mobile and desktop, and states the measured value along with the target value in the finding, so your agency has a clear target.
Why is there no field data for the Speed Index?
The Speed Index is produced by the test run recording the screen build-up and calculating the fill level from the individual frames. That's only possible in a controlled lab — real visitors' screens aren't recorded. That's why the Speed Index is a lab-only metric, while values like LCP or INP can also come from real visitor data.
How can I improve my website's Speed Index?
Make sure the loading order is right: the upper, immediately visible area of the page has to come first. Images further down can load only when scrolled to, scripts for chat, analytics, or ads belong after the content. Smaller image files and a faster server response also help, since they push the entire build forward.