By optimizing the critical rendering path we can significantly improve the time to first render of our pages. Further, understanding the critical rendering path will also serve as a foundation for building well performing interactive applications. It turns out, the process for processing interactive updates is the same, just done in a continuous loop and ideally at 60 frames per second! However, let’s not get ahead of ourselves just yet. First, let’s take a quick, ground-up overview of how the browser goes about displaying a simple page.
Udacity: Website Performance Optimization