test.php

<?php
add_action( 'init', function() {
	$total = 0;

	for ( $i = 1; $i <= 500; $i++ ) {
		$query = new WP_Query( array(
			'posts_per_page' => 1,
			'offset' => $i,
		) );

		$query->the_post();

		$start = microtime( true );
		get_next_post();
		get_previous_post();
		$total += microtime( true ) - $start;
	}

	printf( "%.2f ms", $total * 1000 );
	die();
});