Performance Score
IntermediateA performance score is a single number (usually 0 to 100) that folds several speed metrics into one weighted grade, so a page's overall standing can be grasped at a glance.
A performance score is good for communication — one number a manager and a developer can agree on — but it carries two dangers. First, a composite grade hides the root cause: a 60 does not say whether LCP or a heavy script is to blame; to act you must return to the underlying metrics. Second, the weights are a choice, not a fact, and they change between tool versions; this makes a fixed page's score shift on a tool update without anything on the site changing. More importantly, this score is usually a lab measure from a controlled run, so it does not equal the 75th-percentile experience of real users. Read a performance score as a communication summary, not the basis for technical diagnosis.
How it is calculated
Performance score = the weighted sum of the individual speed-metric scores (such as FCP, LCP, Speed Index, visual stability), mapped onto a 0-to-100 range
The output is a unitless grade. The weights differ between tool versions and the score is usually a lab measure, so it does not equal the field experience of real users.