Add (<link rel="">) preconnect, preload and prefetch resource hints to Magento 2's head

etc-frontend-events.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * @author   : Daan van den Bergh
 * @url      : https://daan.dev
 * @package  : Dan0sz/ResourceHints
 * @copyright: (c) 2019 Daan van den Bergh
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="layout_generate_blocks_after">
        <observer name="dan0sz_resource_hints_framework_view_layout_builder" instance="Dan0sz\ResourceHints\Observer\Framework\View\Layout\Builder" />
    </event>
</config>

etc-module.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * @author   : Daan van den Bergh
 * @url      : https://daan.dev
 * @package  : Dan0sz/ResourceHints
 * @copyright: (c) 2019 Daan van den Bergh
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Dan0sz_ResourceHints" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Catalog" />
        </sequence>
    </module>
</config>

observer-framework-view-layout-builder.php

<?php
/**
 * @author   : Daan van den Bergh
 * @url      : https://daan.dev
 * @package  : Dan0sz/ResourceHints
 * @copyright: (c) 2019 Daan van den Bergh
 */

namespace Dan0sz\ResourceHints\Observer\Framework\View\Layout;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\View\Page\Config as PageConfig;

class Builder implements ObserverInterface
{
    /** @var PageConfig $pageConfig */
    private $pageConfig;

    /**
     * Builder constructor.
     *
     * @param PageConfig  $pageConfig
     */
    public function __construct(
        PageConfig $pageConfig
    ) {
        $this->pageConfig  = $pageConfig;
    }

    /**
     * @param Observer $observer
     *
     * @return $this
     */
    public function execute(Observer $observer)
    {
        $resourceHints = [
            'google' => [
                'resource' => 'https://www.google-analytics.com',
                'type'     => 'preconnect',
                ],
            'next-page' => [
                'resource' => 'https://example.com/next-page',
                'type'     => 'prefetch'
                ],
            'stylesheet' => [
                'resource' => 'styles.css',
                'type'     => 'preload',
                'as'       => 'stylesheet'
                ]
        ];

        foreach ($resourceHints as $resource) {
            $this->pageConfig->addRemotePageAsset(
                $resource['resource'],
                'link_rel',
                [
                    'attributes' => ['rel' => $resource['type'] ]
                ]
            );
        }

        return $this;
    }
}

registration.php

<?php
/**
 * @author   : Daan van den Bergh
 * @url      : https://daan.dev
 * @package  : Dan0sz/ResourceHints
 * @copyright: (c) 2019 Daan van den Bergh
 */

\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Dan0sz_ResourceHints',
    __DIR__
);