Some example use-cases for the Server Timing API in the Performance Lab plugin (see https://github.com/WordPress/performance/pull/553). A previous version for an older implementation of the API can be found at https://gist.github.com/felixarntz/c343a33242b6527ad3f446c8f7f4fec2.

perflab-server-timing-examples.php

<?php
add_action(
	'plugins_loaded',
	function() {
		// Utility function that returns a measuring callback for an action hook,
		// which should be registered as a metric within the action as early as possible
		// (see PHP_INT_MIN usage below).
		$get_measure_action_callback = function( $action_hook ) {
			return function( $metric ) use ( $action_hook ) {
				$metric->measure_before();
				add_action( $action_hook, array( $metric, 'measure_after' ), PHP_INT_MAX );
			};
		};

		// Measure execution time of various critical WordPress action hooks.
		$actions_to_measure = array(
			'plugins-loaded'    => 'plugins_loaded',
			'after-setup-theme' => 'after_setup_theme',
			'init'              => 'init',
			'loaded'            => 'wp_loaded',
			'template-redirect' => 'template_redirect',
		);
		if ( perflab_server_timing_use_output_buffer() ) {
			$actions_to_measure['head']   = 'wp_head';
			$actions_to_measure['footer'] = 'wp_footer';
		}

		foreach ( $actions_to_measure as $metric_slug => $action_hook ) {
			$measure_action_callback = $get_measure_action_callback( $action_hook );

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

		// Measure the 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
		);

		// Measure the 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;
								},
								PHP_INT_MIN
							);
						},
						'access_cap'       => 'exist',
					)
				);
			},
			PHP_INT_MAX
		);
	},
	0
);