diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4e29392..65922b4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - new format of hook events
 - sidebar tracking
 - list of callbacks to hook events
+- Server Timing output for Ajax and REST APIs
 
 ### Changed
 - required PHP version to 7.1
diff --git a/src/Manager/Server_Timing_Manager.php b/src/Manager/Server_Timing_Manager.php
new file mode 100644
index 0000000..eb5593c
--- /dev/null
+++ b/src/Manager/Server_Timing_Manager.php
@@ -0,0 +1,71 @@
+<?php
+declare( strict_types=1 );
+
+namespace Rarst\Laps\Manager;
+
+use Rarst\Laps\Plugin;
+use Rarst\Laps\Record\Record_Interface;
+
+/**
+ * Implements Server Timing API headers.
+ */
+class Server_Timing_Manager {
+
+	/** @var Plugin */
+	private $laps;
+
+	/**
+	 * @param Plugin $laps Container instance.
+	 */
+	public function __construct( Plugin $laps ) {
+
+		$this->laps = $laps;
+
+		add_action( 'admin_init', [ $this, 'send_timing_header' ], PHP_INT_MAX );
+		add_action( 'rest_pre_serve_request', [ $this, 'send_timing_header' ] );
+	}
+
+	/**
+	 * Sends Server Timing API header with records data.
+	 *
+	 * @param mixed $input Pass-through input for attaching to a filter hook.
+	 *
+	 * @return mixed
+	 */
+	public function send_timing_header( $input ) {
+
+		if ( headers_sent() ) {
+			return $input;
+		}
+
+		$ajax = defined( 'DOING_AJAX' ) && DOING_AJAX;
+		$rest = defined( 'REST_REQUEST' ) && REST_REQUEST;
+
+		if ( ! $ajax && ! $rest ) {
+			return $input;
+		}
+
+		// This will be false in REST API w/o nonce, even with valid cookie. Might make sense to filter to true in dev.
+		if ( ! apply_filters( 'laps_can_see', current_user_can( 'manage_options' ) ) ) {
+			return $input;
+		}
+
+		$records = $this->laps['records'];
+		$header  = '';
+
+		/** @var Record_Interface $record */
+		foreach ( $records as $record ) {
+			$duration = $record->get_duration() * 1000;
+
+			if ( $duration < 1 ) {
+				continue;
+			}
+
+			$header .= sprintf( '%s;dur=%d;desc="%s", ', $record->get_category(), $duration, $record->get_name() );
+		}
+
+		header( 'Server-Timing: ' . $header );
+
+		return $input;
+	}
+}
diff --git a/src/Plugin.php b/src/Plugin.php
index 4af88ba..c392b85 100644
--- a/src/Plugin.php
+++ b/src/Plugin.php
@@ -68,10 +68,6 @@ class Plugin extends Container {
 	 */
 	public function run(): void {
 
-		if ( \defined( 'DOING_AJAX' ) && DOING_AJAX ) {
-			return;
-		}
-
 		foreach ( $this->providers as $provider ) {
 
 			if ( $provider instanceof Bootable_Provider_Interface ) {
diff --git a/src/Provider/Manager_Provider.php b/src/Provider/Manager_Provider.php
index 78d4b2d..ac5760f 100644
--- a/src/Provider/Manager_Provider.php
+++ b/src/Provider/Manager_Provider.php
@@ -5,6 +5,7 @@ namespace Rarst\Laps\Provider;
 
 use Pimple\Container;
 use Pimple\ServiceProviderInterface;
+use Rarst\Laps\Manager\Server_Timing_Manager;
 use Rarst\Laps\Plugin;
 use Rarst\Laps\Manager\Asset_Manager;
 use Rarst\Laps\Manager\Load_Order_Manager;
@@ -25,6 +26,7 @@ class Manager_Provider implements ServiceProviderInterface, Bootable_Provider_In
 				new Load_Order_Manager(),
 				new Asset_Manager(),
 				new Toolbar_Manager( $laps ),
+				new Server_Timing_Manager( $laps ),
 			];
 		};
 	}