What does a website performance audit measure?
It measures five things: the Lighthouse performance score on mobile (weight 2) and on desktop (weight 1), plus three lab metrics, Largest Contentful Paint (1.5), Cumulative Layout Shift (1) and Total Blocking Time (1). LCP earns full credit at 2500 ms or faster and scores zero at 4000 ms or slower. The five weights add up to 6.5, so the mobile Lighthouse score alone is roughly a third of the category.
The two Lighthouse scores are converted straight across: a 0 to 100 category score becomes a 0 to 1 fraction, so a Lighthouse performance score of 62 contributes 0.62. Mobile is weighted twice as heavily as desktop because that is the run most sites fail.
The three metric checks use linear partial credit between a good bound and a poor bound. LCP is scored between 2500 ms and 4000 ms, CLS between 0.10 and 0.25, and TBT between 200 ms and 600 ms. Anything at or better than the good bound scores 1.0, anything at or worse than the poor bound scores 0.0, and everything in between lands on the straight line. An LCP of 3250 ms sits exactly halfway, so it scores 0.5.
The metrics come from the mobile Lighthouse run. Desktop metrics are used only when the mobile run is missing entirely. That means the LCP, CLS and TBT you see are phone numbers, not laptop numbers, unless the mobile run failed.
A check with no observation is skipped, not failed. If PageSpeed returns nothing for desktop, the desktop check is dropped from both the numerator and the denominator, and the category is scored on whatever is left. If no check can be evaluated, the category is reported as unmeasured and left out of the overall score rather than counted as a zero. Performance carries a weight of 1.15 in the composite score, tied with SEO for second highest.
What is measured, and how it is scored
5 checks make up the Performance score. Weights are relative within the category and sum to 6.50.
| Check | Rule | Weight |
|---|---|---|
| Lighthouse performance score for the mobile run | The 0 to 100 Lighthouse performance score divided by 100. A score of 100 contributes 1.0, a score of 45 contributes 0.45. Skipped when the mobile run produced no score. | 2 |
| Lighthouse performance score for the desktop run | Same conversion as the mobile score, 0 to 100 divided by 100. Skipped when the desktop run produced no score. | 1 |
| Largest Contentful Paint, the time until the biggest visible element renders | Full credit at 2500 ms or faster. Zero at 4000 ms or slower. Straight-line partial credit in between, so 3250 ms scores 0.5. | 1.5 |
| Cumulative Layout Shift, how much the page jumps while it loads | Full credit at 0.10 or below. Zero at 0.25 or above. Straight-line partial credit in between, so 0.175 scores 0.5. CLS is unitless. | 1 |
| Total Blocking Time, how long the main thread was too busy to respond | Full credit at 200 ms or below. Zero at 600 ms or above. Straight-line partial credit in between, so 400 ms scores 0.5. | 1 |
Performance questions
What LCP counts as good?+
2500 ms or faster earns full credit in this audit. Between 2500 ms and 4000 ms the check loses credit in a straight line, and 4000 ms or slower scores zero. These are Google's published Core Web Vitals bounds, not thresholds invented for this tool.
Why is the mobile Lighthouse score weighted more than desktop?+
Mobile carries weight 2 and desktop carries weight 1, so mobile counts twice as much. Mobile runs use a throttled connection and a slower CPU profile, which is closer to how most visitors actually load a page.
What happens to the performance score if Lighthouse fails to run?+
The affected checks are skipped rather than scored as zero, and the category is scored from whatever was measured. If nothing at all could be measured, Performance is marked unmeasured and excluded from the overall score, instead of dragging it to zero.
Is Total Blocking Time the same as Interaction to Next Paint?+
No. This audit scores Total Blocking Time, a lab metric from the Lighthouse run, against bounds of 200 ms and 600 ms. Interaction to Next Paint is a field metric collected from real users and is not one of the five performance checks.
See how your own page scores on Performance and the other eight categories. One full audit every 7 days is free, no card needed.
Run a free audit