Preload Key Requests
Lining up files to be requested simultaneously in the critical request chain can make your page load faster, according to Google.
Preload improves performance and gives more detailed control to developers.
In the chain posted here in Google's example, the browser is instructed to wait until the app.js script is download, parsed and executed before performing the requests for the ui.js and the styles.css files. The page won't render until all three files are executed.
How to preload the requests
One way to speed up this process is to preload links in the HTML code, alerting the browser to download those key files as quickly as possible. Performing this operation for css files and font files, which are often consider third-level requests, enhances page load.
For more information on different browsers and preload options check: