diff --git a/plugins/performance-lab/includes/server-timing/class-perflab-server-timing-metric.php b/plugins/performance-lab/includes/server-timing/class-perflab-server-timing-metric.php
index fc01e682c3..8f23c9e86f 100644
--- a/plugins/performance-lab/includes/server-timing/class-perflab-server-timing-metric.php
+++ b/plugins/performance-lab/includes/server-timing/class-perflab-server-timing-metric.php
@@ -30,13 +30,21 @@ class Perflab_Server_Timing_Metric {
 	private $value;
 
 	/**
-	 * The value measured before relevant execution logic in seconds, if used.
+	 * The value measured before relevant execution logic in microseconds, if used.
 	 *
 	 * @since 1.8.0
 	 * @var float|null
 	 */
 	private $before_value;
 
+	/**
+	 * The metric description, if used.
+	 *
+	 * @since n.e.x.t
+	 * @var string|null
+	 */
+	private $description;
+
 	/**
 	 * Constructor.
 	 *
@@ -109,6 +117,50 @@ public function get_value() {
 		return $this->value;
 	}
 
+	/**
+	 * Sets the start time.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param float $value Start time in microseconds.
+	 */
+	public function set_start_time( float $value ): void {
+		$this->before_value = $value;
+	}
+
+	/**
+	 * Returns the start time if set.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return float|null Start time if set.
+	 */
+	public function get_start_time(): ?float {
+		return $this->before_value;
+	}
+
+	/**
+	 * Sets the metric description.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string $description Description.
+	 */
+	public function set_description( string $description ): void {
+		$this->description = $description;
+	}
+
+	/**
+	 * Returns the metric description.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return string|null Description if set.
+	 */
+	public function get_description(): ?string {
+		return $this->description;
+	}
+
 	/**
 	 * Captures the current time, as a reference point to calculate the duration of a task afterward.
 	 *
@@ -118,7 +170,7 @@ public function get_value() {
 	 * @since 1.8.0
 	 */
 	public function measure_before(): void {
-		$this->before_value = microtime( true );
+		$this->set_start_time( microtime( true ) );
 	}
 
 	/**
diff --git a/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php b/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php
index 3eed6ad703..ad20824256 100644
--- a/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php
+++ b/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php
@@ -288,19 +288,43 @@ function ( string $output, ?int $phase ): string {
 	 */
 	private function format_metric_header_value( Perflab_Server_Timing_Metric $metric ): ?string {
 		$value = $metric->get_value();
-
-		// If no value is set, make sure it's just passed through.
-		if ( null === $value ) {
-			return null;
-		}
+		$start = $metric->get_start_time();
+		$desc  = $metric->get_description();
 
 		if ( is_float( $value ) ) {
 			$value = round( $value, 2 );
 		}
 
+		if ( is_float( $start ) ) {
+			$start = round( $start, 2 ) * 1000.00;
+		}
+
+		// If no value is set, make sure it's just passed through.
+		if ( null === $start && null === $value ) {
+			return null;
+		}
+
 		// See https://github.com/WordPress/performance/issues/955.
 		$name = preg_replace( '/[^!#$%&\'*+\-.^_`|~0-9a-zA-Z]/', '-', $metric->get_slug() );
 
-		return sprintf( 'wp-%1$s;dur=%2$s', $name, $value );
+		if ( 'response-start' !== $name && 'response-end' !== $name ) {
+			$name = 'wp-' . $name;
+		}
+
+		$header_value = sprintf( '%s', $name );
+
+		if ( $value ) {
+			$header_value .= sprintf( ';dur=%s', $value );
+		}
+
+		if ( $start ) {
+			$header_value .= sprintf( ';start=%s', $start );
+		}
+
+		if ( $desc ) {
+			$header_value .= sprintf( ';desc=%s', $desc );
+		}
+
+		return $header_value;
 	}
 }
diff --git a/plugins/performance-lab/includes/server-timing/defaults.php b/plugins/performance-lab/includes/server-timing/defaults.php
index da7c8d3426..3a8838ccb3 100644
--- a/plugins/performance-lab/includes/server-timing/defaults.php
+++ b/plugins/performance-lab/includes/server-timing/defaults.php
@@ -31,6 +31,7 @@ function perflab_register_default_server_timing_before_template_metrics(): void
 				'measure_callback' => static function ( $metric ): void {
 					// The 'timestart' global is set right at the beginning of WordPress execution.
 					$metric->set_value( ( microtime( true ) - $GLOBALS['timestart'] ) * 1000.0 );
+					$metric->set_start_time( $GLOBALS['timestart'] );
 				},
 				'access_cap'       => 'exist',
 			)
@@ -135,6 +136,27 @@ static function (): void {
 					'measure_callback' => static function ( $metric ): void {
 						// The 'timestart' global is set right at the beginning of WordPress execution.
 						$metric->set_value( ( microtime( true ) - $GLOBALS['timestart'] ) * 1000.0 );
+						$metric->set_start_time( $GLOBALS['timestart'] );
+					},
+					'access_cap'       => 'exist',
+				)
+			);
+
+			perflab_server_timing_register_metric(
+				'response-start',
+				array(
+					'measure_callback' => static function ( $metric ): void {
+						$metric->set_start_time( $GLOBALS['timestart'] );
+					},
+					'access_cap'       => 'exist',
+				)
+			);
+
+			perflab_server_timing_register_metric(
+				'response-end',
+				array(
+					'measure_callback' => static function ( $metric ): void {
+						$metric->set_start_time( microtime( true ) );
 					},
 					'access_cap'       => 'exist',
 				)
@@ -170,6 +192,7 @@ static function ( $acc, $query ) {
 								0.0
 							);
 							$metric->set_value( ( $total_query_time - $GLOBALS['perflab_query_time_before_template'] ) * 1000.0 );
+							$metric->set_start_time( $GLOBALS['perflab_query_time_before_template'] );
 						},
 						'access_cap'       => 'exist',
 					)