My previous blogpost was about the release of the Hybrid Placeholder. It has a getting started video, but it only showed how it can be used. I now released a video in which I will explain why I created the Hybrid Placeholder for Sitecore JSS. After that I will also go through all the files to show how it works.
You can watch the video here: https://youtu.be/krD3ZVfirB4
There was one thing that I forgot to mention in the video. The second request which fetches the heavy parts async uses existing JSS APIs: https://github.com/jbreuer/Hybrid-Placeholder/blob/main/HybridPlaceholder/UserInterface/HybridPlaceholder.jsx#L79