MU Plugin to enable or disable speculative loading introduced in WordPress 6.8.

lr-speculative-loading-toggle.php

<?php
/**
 * Plugin Name: LR Speculative Loading Toggle (UI)
 * Description: Adds admin UI to enable or disable speculative loading introduced in WordPress 6.8.
 * Version: 1.5
 * Author: Luis Martinez
 * Author URI: https://www.lessrain.com
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit; // Prevent direct access
}

/**
 * Initialize default setting (disabled = true) on first load
 */
add_action( 'init', function() {
    if ( get_option( 'lr_disable_speculative_loading' ) === false ) {
        add_option( 'lr_disable_speculative_loading', true );
    }
} );

/**
 * Apply the speculative loading filter
 * Must return null to disable speculative loading (per WP core)
 */
add_filter( 'wp_speculation_rules_configuration', 'lr_speculative_loading_filter', 1 );
function lr_speculative_loading_filter( $config ) {
    $disabled = get_option( 'lr_disable_speculative_loading', true );
    return $disabled ? null : $config;
}

/**
 * Add admin settings page under "Settings"
 */
add_action( 'admin_menu', function() {
    add_options_page(
        'LR Speculative Loading',
        'LR Speculative Loading',
        'manage_options',
        'lr-speculative-loading-settings',
        'lr_render_speculative_loading_settings_page'
    );
} );

/**
 * Register the setting with strict boolean sanitization
 */
add_action( 'admin_init', function() {
    register_setting(
        'lr_speculative_loading_settings_group',
        'lr_disable_speculative_loading',
        [
            'type'              => 'boolean',
            'sanitize_callback' => 'rest_sanitize_boolean',
            'default'           => true,
        ]
    );
} );

/**
 * Render the admin settings page UI
 */
function lr_render_speculative_loading_settings_page() {
    $is_disabled = get_option( 'lr_disable_speculative_loading', true );
    ?>
    <div class="wrap" style="max-width: 900px;">
        <h1 style="margin-bottom:1rem;">LR Speculative Loading</h1>
        <form method="post" action="options.php">
            <?php settings_fields( 'lr_speculative_loading_settings_group' ); ?>
            <?php do_settings_sections( 'lr_speculative_loading_settings_group' ); ?>

            <fieldset>
                <label for="lr_disable_speculative_loading">
                    <input type="checkbox"
                           id="lr_disable_speculative_loading"
                           name="lr_disable_speculative_loading"
                           value="1"
                           <?php checked( 1, $is_disabled ); ?> />
                    Disable speculative preloading
                </label>
                <p class="description">
                    Prevent WordPress from injecting speculative preloading scripts. Enabled by default.
                </p>
            </fieldset>

            <?php submit_button(); ?>
        </form>
    </div>
    <?php
}