What does the Speed index measure?

The Speed Index is a significant metric in web performance that measures the visual loading experience of a webpage. Specifically, it assesses how quickly the contents of a page become visible to the user. By focusing on the progress of loading between specific markers, it captures the timing of how a page renders visually as the browser loads various elements. This means that a lower Speed Index indicates that the content is becoming visible more quickly, improving the perceived performance from the user's perspective.

In contrast, other metrics listed serve different purposes. For example, measuring the time from the first byte to full page load focuses on the overall loading duration rather than the visual rendering sequence. Counting the number of elements loaded on the page does not provide insight into user experience or loading effectiveness. Lastly, the average time users spend on a page relates to user engagement rather than the speed of the visual loading process. Thus, the Speed Index is distinctly about the progression of visual loading, making the choice concerning specific markers the correct interpretation of this metric.