My xhprof/uprofiler setup, tweaked for WordPress and more easily profiling segments.

append.php

<?php

use Rarst\Profiler\Handler;

global $wp;

if ( Handler::$profiling && empty( $wp ) ) {
	Handler::close();
}

class-handler.php

<?php
namespace Rarst\Profiler;

class Handler {

	public static $profiling = false;

	public static $mode;

	public static $includes = [
		'xhprof'    => [
			'../xhprof/xhprof_lib/utils/xhprof_lib.php',
			'../xhprof/xhprof_lib/utils/xhprof_runs.php',
		],
		'uprofiler' => [
			'../uprofiler/uprofiler_lib/utils/uprofiler_lib.php',
			'../uprofiler/uprofiler_lib/utils/uprofiler_runs.php',
		],
	];

	public static $hosts = [
		'xhprof'    => 'http://xhprof.rarst.net/',
		'uprofiler' => 'http://uprofiler.rarst.net/',
	];

	static function run() {

		self::$profiling = isset( $_REQUEST['XHPROF'] ) || isset( $_REQUEST['UPROFILER'] ) || isset( $_REQUEST['XDEBUG_PROFILE'] );

		define( 'PROFILING', self::$profiling );

		if ( ! self::$profiling ) {
			return;
		}

		if ( isset( $_REQUEST['XDEBUG_PROFILE'] ) ) {
			self::$mode = 'xdebug';

			return;
		}

		self::$mode = isset( $_REQUEST['XHPROF'] ) ? 'xhprof' : 'uprofiler';

		foreach ( self::$includes[ self::$mode ] as $file ) {
			require_once $file;
		}

		global $wp_filter;

		if ( ! isset( $wp_filter ) ) {
			$wp_filter = array();
		}

		$callback                   = __CLASS__ . '::close';
		$wp_filter['shutdown'][100] = array(
			$callback => array(
				'function'      => $callback,
				'accepted_args' => 1,
			)
		);

		self::open();
	}

	static function open( $name = '' ) {

		if ( ! self::$profiling || ! self::is_capturing( $name ) ) {
			return;
		}

		switch ( self::$mode ) {
			case 'xhprof' :
				$flags = version_compare( PHP_VERSION, '5.5', '>=' ) ? XHPROF_FLAGS_NO_BUILTINS : XHPROF_FLAGS_MEMORY;
				xhprof_enable( $flags );
				break;

			case 'uprofiler':
				$flags = UPROFILER_FLAGS_MEMORY;
				uprofiler_enable( $flags );
				break;
		}
	}

	static function close( $name = '' ) {

		if ( ! self::$profiling || ! self::is_capturing( $name ) ) {
			return;
		}

		switch ( self::$mode ) {
			case 'xhprof':
				$data = xhprof_disable();
				$runs = new \XHProfRuns_Default();
				break;

			case 'uprofiler':
				$data = uprofiler_disable();
				$runs = new \UprofilerRuns_Default();
				break;
		}

		$namespace    = $_SERVER['SERVER_NAME'];
		$run_id       = $runs->save_run( $data, $namespace );
		$notification = <<<NOTIFICATION
<div style="position:fixed; right: 5px; bottom: 5px;">
<a href="%s" target="_blank" class="btn btn-success" style="margin-right: 5px;">Profiler output</a>
<a href="%s" target="_blank" class="btn btn-info">Callgraph</a>
</div>
NOTIFICATION;

		$profiler_url  = self::get_url( 'index.php?run=%s&source=%s', $run_id, $namespace );
		$callgraph_url = self::get_url( 'callgraph.php?run=%s&source=%s', $run_id, $namespace );
		printf( $notification, $profiler_url, $callgraph_url );
	}

	static function is_capturing( $name ) {
		return $_REQUEST[ strtoupper( self::$mode ) ] === $name;
	}

	static function get_url( $path = '', $run_id = '', $namespace = '' ) {
		return self::$hosts[ self::$mode ] . sprintf( $path, $run_id, $namespace );
	}
}

prepend.php

<?php

require '/server/ext/vendor/autoload.php';
require __DIR__ . '/class-handler.php';

\Rarst\Profiler\Handler::run();

if ( defined( 'PROFILING' ) && ! PROFILING ) {

}

uprofiler-bookmarklet.js

javascript:if (document.URL.indexOf('UPROFILER') < 1) {
	var sep = document.URL.indexOf('?');
	sep = (sep < 1) ? '?' : '&';
	window.location.href = document.URL + sep + 'UPROFILER';
}