Add JSDelivr as a prefetch resource provider to speed up page load times in WordPress

functions.php

<?php

add_filter( 'wp_resource_hints', 'your_add_resource_hints', 10, 2 );

/**
 * Try to improve performance with resource hints.
 *
 * WP will output protocol-relative 'dns-prefetch' for all scripts and styles which are enqueued
 * from external hosts.
 * WP will not do 'preconnect', 'prefetch', or 'prerender' automatically.
 *
 * @link https://make.wordpress.org/core/2016/07/06/resource-hints-in-4-6/
 * @link https://www.keycdn.com/blog/resource-hints/
 * @link https://www.machmetrics.com/speed-blog/guide-to-browser-hints-preload-preconnect-prefetch/
 * @link https://caniuse.com/#search=preconnect
 * @link https://www.igvita.com/2015/08/17/eliminating-roundtrips-with-preconnect/
 * @link https://core.trac.wordpress.org/ticket/38121
 * @link https://w3c.github.io/resource-hints/#dfn-preconnect
 * @link https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content#Cross-origin_fetches
 * @link https://html.spec.whatwg.org/multipage/urls-and-fetching.html#cors-settings-attributes
 * @link https://wordpress.org/plugins/pre-party-browser-hints/
 *
 * @param $hints
 * @param $relation_type
 *
 * @return array
 */
function your_add_resource_hints( $hints, $relation_type ) {
	if ( 'preconnect' === $relation_type ) {
		$hints[] = [
			'href'        => 'https://cdn.jsdelivr.net/',
			'crossorigin' => 'anonymous',
		];
	}

	return $hints;
}

I don't know if crossorigin is actually helping you. Unless you are loading fonts inside the css.

Will need to make the choice for each implementation example, but here's an example from https://getbootstrap.com/

<!-- CSS only -->
<link
href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css"
rel="stylesheet"
integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3"
crossorigin="anonymous">

Ah yes. But with a preconnect link it's another story. Sorry my bad.

<link rel="preconnect" href="https://...." crossorigin>

This is another type of crossorigin attribute.

No sweat, thanks for contributing :)