LUX measures CPU using the Long Tasks API. A "long task" is defined as a browser event that takes more than 50ms to execute. LUX shows the total of all long tasks, the longest long task, and number of long tasks in the page. In this example, an improvement made on March 22nd shaved off more than one second of CPU time:
Long tasks make pages feel janky. Two metrics that capture jankiness are shown in the LUX chart below:
The list of Top Errors helps you identify the errors that are most affecting customers. The LUX Errors dashboard also shows the distribution of errors across top browsers, regions, and pages. You can click on any error message to drill down on it.
Note that "Script error" is typically from cross origin scripts where error details are not available. This can happen for third party scripts but also for scripts you own that are served from a different domain, such as a CDN. To see the error details you must include the crossorigin SCRIPT attribute and the Access-Control-Allow-Origin response header.
The details for individual errors makes it easy to find and fix problems. The name of the offending script is given, or "HTML document" if the error occurred in an inline script. The line and column number help track down exactly where the error occurs.
If you're already a LUX user, these JS dashboards are available in your LUX menu. If you're not using LUX, give it a try for free.