embolden/wp-menu-cache.php

## wp-menu-cache.php
<?php
function _s_nav_menu( $menu_options ) {
	$transient_name = $menu_options['theme_location'] . '-menu-cache';
	$menu           = null;
	$echo           = isset( $menu_options['echo'] ) ? $menu_options['echo'] : true;

	if( $menu = get_transient( $transient_name ) ) {
		// Do nothing; jut assign the transient to $menu
	}else {
		// Reset the echo value; we need to return to set
		$menu_options['echo'] = 0;
		$menu = wp_nav_menu( $menu_options );
		set_transient( $transient_name, $menu, DAY_IN_SECONDS );
	}

	if( $echo ) {
		echo $menu;
	}else {
		return $menu;
	}
}

function _s_clear_nav_menu_transient( $menu_id ) {
	$theme_locations = get_nav_menu_locations();

	if( $keys = array_keys( $theme_locations, $menu_id ) ) {
		foreach( $keys as $key ) {
			$transient_name = $key . '-menu-cache';
			delete_transient( $transient_name );
		}
	}
}
add_action( 'wp_update_nav_menu', '_s_clear_nav_menu_transient' );