Collected from: Video analysis
This metric is measured from the video analysis of the web page's loading. Start Render gives the necessary time for the first element to render on the user's screen (the web page is not totally blank any longer). Considering that the video analysis only concerns the visible part of the tested web page (above the foldline).
Please note that the first element to render isn't necessarily a significant one (it may be a minor element as a background color, a text or anything else).