Read full writeup: https://weston.ruter.net/2025/03/07/eagerly-prerender-key-urls-with-speculative-loading-in-wordpress/

speculative-loading-prerender-homepage-from-singular-template.php

<?php
/**
 * Plugin Name: Speculative Loading: Eagerly Prerender Homepage from Singular Template
 * Plugin URI: https://gist.github.com/westonruter/eb51e56c7e92f3cd6e3088eb87d9ce07
 * Description: When viewing a singular post or page, eagerly prerender the homepage so that navigating there will be instant.
 * Requires at least: 5.7
 * Requires PHP: 7.2
 * Version: 0.1.0
 * Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * License: GPLv2 or later
 * License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 * Update URI: https://gist.github.com/westonruter/eb51e56c7e92f3cd6e3088eb87d9ce07
 * Gist Plugin URI: https://gist.github.com/westonruter/eb51e56c7e92f3cd6e3088eb87d9ce07
 *
 * @package SpeculationRulesPrerenderHomepageFromSingularTemplate
 */

add_action(
	'wp',
	static function (): void {
		// Abort if WordPress is not serving a singular post or page.
		if ( ! is_singular() ) {
			return;
		}

		/*
		 * Create the eager speculation rule for the link to the homepage. This presumes that the link is constructed
		 * with the rel-home Microformat <https://microformats.org/wiki/rel-home> which is used by all core themes.
		 */
		$speculation_rule = array(
			'source'    => 'document',
			'where'     => array(
				'selector_matches' => 'a[rel="home"]',
			),
			'eagerness' => 'eager',
		);

		/*
		 * As an alternative to the above, the speculation rule could instead be constructed as follows:
		 *
		 * array(
		 *     'source'    => 'list',
		 *     'urls'      => array( home_url( '/' ) ),
		 *     'eagerness' => 'eager',
		 * )
		 *
		 * However, there is a danger that a plugin may append query parameters to the links as they are rendered in
		 * the template. This could result in a post permalink being eagerly prerendered which does not correspond
		 * to the actual post link which the user navigates to, and thereby resulting in a wasted the prerender.
		 * An added benefit to using a document source with a selector is that it ensures the URL being prerendered
		 * actually appears in the page as a link: granted it surely would the vast majority of the time, but in
		 * WordPress anything is possible.
		 */

		// Add the new speculation rule to the page.
		if ( class_exists( 'WP_Speculation_Rules' ) ) {
			// This branch is for WordPress 6.8-beta1. See <https://make.wordpress.org/core/2025/03/06/speculative-loading-in-6-8/>.
			add_action(
				'wp_load_speculation_rules',
				static function ( WP_Speculation_Rules $speculation_rules ) use ( $speculation_rule ): void {
					$speculation_rules->add_rule(
						'prerender',
						'prerender-homepage-from-singular-template',
						$speculation_rule
					);
				}
			);
		} else {
			// This branch is for WordPress versions prior to 6.8-beta1.
			// Note that multiple 'speculationrules' scripts can be on a page, so it is fine if the Speculative Loading
			// plugin is also adding its script that may do moderate prerendering for all links as well.
			add_action(
				'wp_footer',
				static function () use ( $speculation_rule ): void {
					wp_print_inline_script_tag(
						(string) wp_json_encode( array( 'prerender' => array( $speculation_rule ) ) ),
						array( 'type' => 'speculationrules' )
					);
				}
			);
		}
	}
);