That’s not to say you have an excuse not to do these things — they’re all best-practice, and they all help.
But they don’t represent the whole site speed picture.
In fact, Moz has a popular article on this subject: How to Achieve 100/100 with the Google Page Speed Test Tool. I want to make three points: I’m writing with SEO practitioners in mind. You should walk away with enough perspective to start asking the right questions.
Some aren’t covered at all by Page Speed Insights, and some are only covered halfway: The screenshots in this post are created with Chrome Dev Tools.
If that URL corresponded to a webpage, the browser will usually discover that it needs to load more resources to render the page. It must recursively go through the same steps listed above to load each of these files.
Fortunately, once a server has been found (“DNS Lookup” in the image above), the browser won’t need to look it up again.
The first three are independent of file size; they are effectively constant costs.
These costs are incurred with each request regardless of whether the payload is a tiny, minified CSS file or a huge uncompressed image.