Mini WordPress plugin that uses the Server Timing API of the Performance Lab plugin (also see https://github.com/WordPress/performance/pull/553) to capture additional metrics

server-timing-extra.php

<?php
/**
 * Plugin Name: Server Timing Extra
 * Plugin URI: https://gist.github.com/felixarntz/63c05392dbf7d51cc7f8f4a424b1ff39
 * Description: Exposes additional Server-Timing metrics using the Performance Lab plugin's Server Timing API.
 * Requires at least: 6.1
 * Requires PHP: 5.6
 * Version: 0.1.0
 * Author: Felix Arntz
 * Author URI: https://felix-arntz.me
 * License: GPLv2 or later
 * License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 * Text Domain: server-timing-extra
 *
 * @package server-timing-extra
 */

/**
 * Measures the total time for a given action hook.
 *
 * @param string $hookname The action hook name.
 */
function stx_measure_action( $hookname ) {
	$metric_slug = stx_hookname_to_metric_slug( $hookname );

	$measure_callback = function( $metric ) use ( $hookname ) {
		$metric->measure_before();
		add_action( $hookname, array( $metric, 'measure_after' ), PHP_INT_MAX, 0 );
	};

	// If action is already running, at least measure the remainder.
	if ( doing_action( $hookname ) ) {
		perflab_server_timing_register_metric(
			$metric_slug,
			array(
				'measure_callback' => $measure_callback,
				'access_cap'       => 'exist',
			)
		);
	} else {
		add_action(
			$hookname,
			function() use ( $metric_slug, $measure_callback ) {
				perflab_server_timing_register_metric(
					$metric_slug,
					array(
						'measure_callback' => $measure_callback,
						'access_cap'       => 'exist',
					)
				);
			},
			defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : -9999
		);
	}
}

/**
 * Measures the total time for a given filter hook.
 *
 * @param string $hookname The filter hook name.
 */
function stx_measure_filter( $hookname ) {
	$metric_slug = stx_hookname_to_metric_slug( $hookname );

	$measure_callback = function( $metric ) use ( $hookname ) {
		$metric->measure_before();
		add_filter(
			$hookname,
			function( $passthrough ) use ( $metric ) {
				$metric->measure_after();
				return $passthrough;
			},
			PHP_INT_MAX
		);
	};

	add_filter(
		$hookname,
		function( $passthrough ) use ( $metric_slug, $measure_callback ) {
			perflab_server_timing_register_metric(
				$metric_slug,
				array(
					'measure_callback' => $measure_callback,
					'access_cap'       => 'exist',
				)
			);
			return $passthrough;
		},
		defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : -9999
	);
}

/**
 * Measures the times for every callback of a given action or filter hook.
 *
 * @param string $hookname The action or filter hook name.
 */
function stx_profile_hook_callbacks( $hookname ) {
	global $wp_filter;

	if ( ! isset( $wp_filter[ $hookname ] ) ) {
		return;
	}

	foreach ( $wp_filter[ $hookname ]->callbacks as $priority => $callbacks ) {
		foreach ( $callbacks as $idx => $callback ) {
			$metric_slug = stx_hookname_to_metric_slug( $hookname ) . '--' . stx_hookname_to_metric_slug( $idx );

			$wp_filter[ $hookname ]->callbacks[ $priority ][ $idx ]['function'] = perflab_wrap_server_timing(
				$wp_filter[ $hookname ]->callbacks[ $priority ][ $idx ]['function'],
				$metric_slug,
				'exist'
			);
		}
	}
}

/**
 * Gets a Server-Timing metric slug name based on the given hook name.
 *
 * @param string $hookname The action or filter hook name.
 * @return string Corresponding metric slug.
 */
function stx_hookname_to_metric_slug( $hookname ) {
	return str_replace( '_', '-', str_replace( 'wp_', '', $hookname ) );
}

/**
 * Defines and configures extra metrics to capture.
 */
function stx_capture_extra_metrics() {
	if ( ! function_exists( 'perflab_server_timing_register_metric' ) ) {
		return;
	}

	/* More actions can be added here. */
	$action_hooks = array(
		'plugins_loaded',
		'after_setup_theme',
		'init',
		'widgets_init',
		'wp_loaded',
		'template_redirect',
	);
	if ( perflab_server_timing_use_output_buffer() ) {
		$action_hooks[] = 'wp_head';
		$action_hooks[] = 'wp_enqueue_scripts';
		$action_hooks[] = 'wp_footer';
	}
	array_walk( $action_hooks, 'stx_measure_action' );

	/* More filters can be added here. */
	$filter_hooks = array();
	array_walk( $filter_hooks, 'stx_measure_filter' );

	// Measure all callbacks of the 'wp_enqueue_scripts' action individually.
	add_action(
		'template_redirect',
		function() {
			if ( perflab_server_timing_use_output_buffer() ) {
				stx_profile_hook_callbacks( 'wp_enqueue_scripts' );
			}
		}
	);

	// Time it takes to parse the request (including, but not exclusively, the 'parse_request' action).
	add_filter(
		'do_parse_request',
		function( $passthrough ) {
			perflab_server_timing_register_metric(
				'parse-request',
				array(
					'measure_callback' => function( $metric ) {
						$metric->measure_before();
						add_action( 'parse_request', array( $metric, 'measure_after' ), PHP_INT_MAX );
					},
					'access_cap'       => 'exist',
				)
			);
			return $passthrough;
		},
		PHP_INT_MAX
	);

	// Time it takes to locate the template.
	add_action(
		'template_redirect',
		function() {
			perflab_server_timing_register_metric(
				'locate-template',
				array(
					'measure_callback' => function( $metric ) {
						$metric->measure_before();
						add_filter(
							'template_include',
							function( $passthrough ) use ( $metric ) {
								$metric->measure_after();
								return $passthrough;
							},
							defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : -9999
						);
					},
					'access_cap'       => 'exist',
				)
			);
		},
		PHP_INT_MAX
	);
}
add_action( 'plugins_loaded', 'stx_capture_extra_metrics', 0 );