SpeedLab Score

The SpeedLab performance score that you see results from performance tests run on real browsers on real devices in real locations. This means none of the tests are run on simulators for any configurations. For the test, the network is throttled using packet-level throttling. We perform five runs for each test and use the median score as the final test score. This approach results in a consistent score for a particular configuration by reducing the variability.

While the configuration and median logic remain the same across browsers, the metrics used to calculate the scores differ as browsers offer different mechanisms for measuring performance.

Chrome and Edge

SpeedLab uses Google's Lighthouse tests to generate reports for Chrome and Edge browsers. Lighthouse uses the browser's performance APIs to fetch different metrics. These metrics are then benchmarked against corresponding metrics from millions of URLs in the HTTP Archive. Each metric is given a certain weightage to measure a balanced mix of different aspects of performance.

The total score is arrived at by combining the scores of individual metrics as per their weightage. The weightage and the component metrics changes over time as the Lighthouse team learns more about user experience and new ways to measure performance are added to browsers.

The current version of Lighthouse is 9.2.0, and the weightage of the metrics is as follows:

Firefox

Lighthouse scores are supported only on Chromium-based browsers. On Firefox, we directly measure the various metrics through the browser's performance APIs. Firefox doesn't offer as many metrics as Chrome but more than any other browser.

The metrics used and their weightage is as follows:

Safari

Safari exposes the least number of performance metrics. As Safari is primarily used in Apple devices that have better hardware compared to other device manufacturers, it leads to relatively better user experience, unless the devices are a few years old.

Derived metrics like Speed Index are not supported on Safari on iOS as the necessary browser API isn't present. So the metrics and weightage used for Safari on iOS and macOS are different.

The metrics used and their weightage is as follows:

MacOS

iOS