This metric is measured from the video analysis of the web page's loading. Speed index transcribes the average display speed of the pixels included in the visible part of the tested web page (above the foldline). Thereby, this indicator takes into account the display progressiveness of the different elements composing the page. The lower is the Speed Index, the fastest your web page displays. You can refer to our detailed article to better understand the speed index.
As for Visually Complete, the Speed Index calculation may be affected by animations.
You may have already heard about Speed Index through the Webpagetest project. Dareboost uses the same algorithm than this project maintained by a Google developer.