The TTFB measures the delay before the user's browser receives the very first data on the page. This essentially corresponds to the processing time required on the server to produce the page. Note that this response time will also have a large impact on your SEO ranking.
That is because it strongly impacts the Google program responsible for indexing pages. Google recommends a server response time of less than a few hundred milliseconds. A "bad" TTFB is a huge handicap for web performance. That is because this is only the first step to loading the page.
That will increase the risk of seeing a surfer abandon his visit. In fact, the Start Render time highlights a real dead time for the user. During that time he is in front of a totally blank page, without content or possible interaction.
The Speed Index is also an indicator calculated from the video analysis of the loading of the tested web page. This is a particularly valuable indicator. In fact, the Speed Index is now recognized as one of the best indicators of the quality of user experience in terms of display speed.
The Speed Index is done in order to account for the speed of display of the page in all its complexity. The Speed Index summarizes all that data in one numerical value. If you only need to monitor one metric, it is the one that will offer you the most comprehensive vision.
You may say, "So these three indicators are enough to track web performance?" In case you have a problem, or if you need to find ways to improve your results, you will need other complementary elements to carry out your investigations. Therefore, you will need to go into the details of the various additional stages of loading