Good little case study on how YouTube optimized their First Contentful Paint and Largest Contentful Paint by applying
fetchpriority to their poster image.
My favorite nugget is that they tested using an actual video thumbnail for their poster image versus a solid black poster image, and the black image performed better in user studies:
Using a solid black poster image showed the best results in user studies. Users found the transition from solid black to the first frame of the video to be a less-jarring experience for autoplay videos.