Speed index is an application performance monitoring metric that is automatically calculated by mabl. It indicates how users are likely to perceive the load speed of your application user interface (UI) as they interact with it on their devices. The lower the score, the better.
Speed index is available only for test runs in Chrome and Edge for steps with a screenshot and step trace. Read on to learn more about speed index.
Speed index chart
How it works
Speed index tracks application page load performance over time in Chrome and Edge. It tracks the loading speed of page URLs and individual steps within a test, even if there is no additional page load. This capability makes it possible to monitor performance of single page apps, where pages are dynamically updated in response to user actions without changing the URL.
To view the speed index of a step in any Chrome or Edge test run, select the step in the output view for the run, click the Logs tab, and then select the Performance tab. This view shows the speed index for updating the page in response to the action in the highlighted step. In addition to the highlighted value for the test run being viewed, the chart shows all recent runs of the same test step for the same test version, plan, and environment.
Speed index chart for a test step
Speed index is a key metric in the Google Lighthouse tools for measuring page loading speed. Note that currently Lighthouse tools do not track loading performance of UI interactions where the page URL doesn't change.
Lowering the speed index
You can lower your speed index score by optimizing the application pages and screens to load visually faster. Some good resources on the topic are the articles on Optimizing Content Efficiency and Optimizing the Critical Rendering Path from the Google Developers network.