Page Speed Insights measures the performance of a webpage. The PageSpeed Score ranges from 0 to 100 points. A higher score is better and a score of 85 or above indicates that the page is performing well. It does so for mobile devices and desktop devices. It fetches the url twice, once with a mobile user-agent, and once with a desktop-user agent.
PageSpeed Insights measures how the page can improve its performance on:
time to above-the-fold load: Elapsed time from the moment a user requests a new page and to the moment the above-the-fold content is rendered by the browser. time to full page load: Elapsed time from the moment a user requests a new page to the moment the page is fully rendered by the browser. PageSpeed Insights only evaluates the network-independent aspects of page performance: the server configuration, the HTML structure of a page, and use of external resources (such as images, JavaScript & CSS). In the end the implementation of the suggestions should improve the relative performance of the page. However, since the performance of a network connection varies considerably again the absolute performance of the page will be dependent upon a user’s network connection.