bjornjohansen/wordpress-menu-cache-timing.php

## wordpress-menu-cache-timing.php
<?php
/**
 * WordPress menu cache timing.
 *
 * @package BJ\Menu
 * @author bjornjohansen
 * @version 0.1.0
 * @license https://www.gnu.org/licenses/old-licenses/gpl-2.0.html  GNU General Public License version 2 (GPLv2)
 */

/**
 * Add the logging timer.
 *
 * @param string|null $output Nav menu output to short-circuit with. Default null.
 * @param stdClass    $args   An object containing wp_nav_menu() arguments.
 * @return string|null Output passthrough (default null).
 */
add_filter( 'pre_wp_nav_menu', function( $output, $args ) {
	global $wp_menu_generation_start;
	$wp_menu_generation_start = microtime( true );
	return $output;
}, 9, 2 );

/**
 * Log the menu generation time if we have a cache hit.
 *
 * @param string|null $output Nav menu output to short-circuit with. Default null.
 * @param stdClass    $args   An object containing wp_nav_menu() arguments.
 * @return string|null Output passthrough (default null).
 */
add_filter( 'pre_wp_nav_menu', function( $output, $args ) {
	if ( ! is_null( $output ) ) {
		global $wp_menu_generation_start;
		$wp_menu_generation_time = microtime( true ) - $wp_menu_generation_start;
		write_log( sprintf( 'Cache hit: Menu output was fetched in %d ms', round( $wp_menu_generation_time * 1000 ) ) );
	}
	return $output;
}, 11, 2 );

/**
 * Log the menu generation time.
 *
 * @param string   $nav_menu The HTML content for the navigation menu.
 * @param stdClass $args     An object containing wp_nav_menu() arguments.
 */
add_filter( 'wp_nav_menu', function( $nav_menu, $args ) {
	global $wp_menu_generation_start;
	$wp_menu_generation_time = microtime( true ) - $wp_menu_generation_start;
	write_log( sprintf( 'Cache miss: Menu output was generated in %d ms', round( $wp_menu_generation_time * 1000 ) ) );
	return $nav_menu;
}, 11, 2 );