What key performance metric measures the time until the visual part of a webpage is completely loaded?

The Speed Index is the key performance metric that measures how quickly the visual content of a webpage is displayed to the user. It quantifies how fast the visible parts of a page are rendered, providing a score that reflects the user's perception of loading time. A lower Speed Index indicates that the visual elements have appeared on the screen more quickly, which correlates with a better user experience.

This metric is particularly valuable because it focuses on user interaction and engagement, rather than just backend loading times or data transfer rates. By prioritizing the visual load time, developers can better understand how users experience the performance of their websites, leading to more intuitive improvements in web design and performance optimization.

In contrast, other metrics do not measure visual readiness. While User Action Duration relates to the measurement of the time taken for a specific user action, Load Event End indicates when the loading of resources concludes, and Time to First Byte measures the time it takes for the server to respond, none of these directly assess the visual loading experience from the user's perspective as Speed Index does.