WP_Batch_Processor::boot();

// Manually define the path constants to eliminate
// possible errors when resolving the paths and also
// include trailing slash at the end.

if ( ! defined('WP_BP_PATH')) {
    define('WP_BP_PATH', '/path/to/wp-content/plugins/your-plugin/libraries/wp-batch-processing/');
}

if ( ! defined('WP_BP_URL')) {
    define('WP_BP_URL', 'https://site.com/wp-content/plugins/your-plugin/libraries/wp-batch-processing/');
}

WP_Batch_Processor::boot();

if ( class_exists( 'WP_Batch' ) ) {

	/**
	 * Class MY_Example_Batch
	 */
	class MY_Example_Batch extends WP_Batch {

		/**
		 * Unique identifier of each batch
		 * @var string
		 */
		public $id = 'email_post_authors';

		/**
		 * Describe the batch
		 * @var string
		 */
		public $title = 'Email Post Authors';

		/**
		 * To setup the batch data use the push() method to add WP_Batch_Item instances to the queue.
		 *
		 * Note: If the operation of obtaining data is expensive, cache it to avoid slowdowns.
		 *
		 * @return void
		 */
		public function setup() {

			$users = get_users( array(
				'number' => '40',
				'role'   => 'author',
			) );

			foreach ( $users as $user ) {
				$this->push( new WP_Batch_Item( $user->ID, array( 'author_id' => $user->ID ) ) );
			}
		}

		/**
		 * Handles processing of batch item. One at a time.
		 *
		 * In order to work it correctly you must return values as follows:
		 *
		 * - TRUE - If the item was processed successfully.
		 * - WP_Error instance - If there was an error. Add message to display it in the admin area.
		 *
		 * @param WP_Batch_Item $item
		 *
		 * @return bool|\WP_Error
		 */
		public function process( $item ) {

			// Retrieve the custom data
			$author_id = $item->get_value( 'author_id' );

			// Return WP_Error if the item processing failed (In our case we simply skip author with user id 5)
			if ( $author_id == 5 ) {
				return new WP_Error( 302, "Author skipped" );
			}

			// Do the expensive processing here.
			// ...

			// Return true if the item processing is successful.
			return true;
		}
		
		/**
		 * Called when specific process is finished (all items were processed).
		 * This method can be overriden in the process class.
		 * @return void
		 */
		public function finish() {
			// Do something after process is finished.
			// You have $this->items, etc.
		}
	}
}

/**
 * Initialize the batches.
 */
function wp_batch_processing_init() {
    $batch = new MY_Example_Batch();
    WP_Batch_Processor::get_instance()->register( $batch );
}
add_action( 'wp_batch_processing_init', 'wp_batch_processing_init', 15, 1 );

function wp_bp_my_custom_delay($delay) {
   return 2; // in seconds
}
add_filter('wp_batch_processing_delay', 'wp_bp_my_custom_delay', 10, 1);