Which type of actions can the speed index be calculated for?

The speed index is a metric that reflects how quickly the contents of a page are visibly populated in a web browser. It is primarily concerned with how fast the visual elements of a page are rendered during the loading process, which directly relates to the overall user experience.

Load actions specifically refer to the initial loading of a webpage, where various elements such as images, scripts, and styles are fetched and displayed to the user. The speed index is calculated during this process, as it measures how quickly significant portions of the page appear to the user. It provides insight into the user's perception of speed when interacting with a webpage that is being loaded, which is why it is particularly relevant for load actions.

When considering the other types of actions like XHR (XMLHttpRequest), performance testing actions, or all actions in general, they do not align with the specific focus of measuring the visual readiness during the initial page load. XHR actions involve background requests made after the page has already loaded, while performance testing actions could encompass a variety of metrics and scenarios that do not pertain directly to the speed index. Thus, it is clear that the speed index is meant to be evaluated in the context of load actions, making this choice the most suitable.