Chrome has a hidden flag for capturing screenshots of your page at various stages of the page loading and rendering process
Damon Bauer writes:
You may have heard of the wonderful site, WebpageTest, when researching ways to improve your site performance. One of the most useful features (in my opinion) is the Filmstrip; it shows you incremental screenshots of the browser rendering your site, so you can see how quickly users are able to start consuming content. As of the past couple of months, you can enable similar functionality in Chrome!
Read more from the source: damonbauer.me