It is useful to specify these different later connections so that the browser anticipates them and improves the loading of the relevant resources when the time comes. Pre-connections therefore improve the page loading time.
Establish early connections with important external sources. To tell the browser which pre-connection to make, it is necessary to add the code at the <head> section of the HTML code.
Example for the subsequent loading of the resource "https://exemple.com/script.js" :
<link rel="preconnect" href="https://exemple.com">