What is measured by the onDOMContentLoaded metric?

The onDOMContentLoaded metric specifically measures the time spent executing handlers that are registered to run when the DOMContentLoaded event is fired. This event signals that the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading. Thus, the metric captures the responsiveness and performance of scripts that need to run immediately after the DOM becomes available, reflecting how quickly the application can begin to provide interactivity to the user after the HTML has been parsed.

While the other options touch on aspects related to page load timing, they do not accurately describe the scope of the onDOMContentLoaded metric. For example, it doesn’t account for the entire page load time (which includes all resources), the user’s wait time for interaction may extend beyond this event, and loading external resources pertains to assets that may not impact the initial DOM readiness reflected by this specific metric.