cron-sleep-test.php

<?php
/**
 * Plugin Name: Cron Sleep Test
 * Description: A 15-second cron job that runs on every page load.
 * Version: 3.1.0
 */

if ( ! defined( 'WP_CRON_LOCK_TIMEOUT' ) ) {
	define( 'WP_CRON_LOCK_TIMEOUT', 1 );
}

add_filter( 'cron_schedules', function ( $schedules ) {
	$schedules['every_second'] = array(
		'interval' => 1,
		'display'  => 'Every Second',
	);
	return $schedules;
} );

add_action( 'init', function () {
	if ( ! wp_next_scheduled( 'cron_sleep_test_event' ) ) {
		wp_schedule_event( time(), 'every_second', 'cron_sleep_test_event' );
	}
} );

add_action( 'cron_sleep_test_event', function () {
	$start = microtime( true );
	error_log( '[Cron Sleep Test] Started at ' . gmdate( 'Y-m-d H:i:s' ) );
	sleep( 15 );
	$elapsed = round( microtime( true ) - $start, 2 );
	error_log( '[Cron Sleep Test] Finished at ' . gmdate( 'Y-m-d H:i:s' ) . " (ran for {$elapsed}s)" );
} );

register_deactivation_hook( __FILE__, function () {
	wp_clear_scheduled_hook( 'cron_sleep_test_event' );
} );