gistfile1.php

<?php

$query_timer = 0;

add_filter( 'do_parse_request', function( $parse ) {
	global $query_timer;

	$query_timer = microtime( true );

	return $parse;
} );

add_action( 'parse_request', function() {
	global $query_timer;

	$time = microtime( true ) - $query_timer;

	add_action( 'wp_head', function() use ( $time ) {
		echo '<!-- timer: ' . $time . ' -->';
	} );
} );