This plugin implemented chunk syncing, so syncing not get blocked in low execution time, only implemented user delete module

user-sync-custom.css

.usc--status {}

.usc--area {
	position: fixed;
	width: 100%;
	height: 100%;
	background: rgba(0, 0, 0, 0.5);
	top: 0;
	left: 0;
}

.usc--wrapper {
	width: 400px;
	background: #fff;
	border-radius: 6px;
	margin: 10% auto;
	padding: 50px 35px;
	box-shadow: 0px 3px 10px rgba(0, 0, 0, 0.18);
	text-align: center;
}

.usc--slider {
	margin: 35px 0px 15px;
	background: #ddd;
	border-radius: 15px;
	text-align: center;
	padding: 5px;
	position: relative;
	overflow: hidden;
}

.usc--slider-inner {
	position: absolute;
	top: 0;
	left: 0;
	height: 100%;
	width: 0%;
	background: #41d85b;
	transition: width .35s ease;
}

.usc--heading {
	margin-top: 0px;
	font-size: 18px;
}

.usc--slider-percentage {
	position: relative;
	z-index: 2;
}

.usc--slider-status {
	margin-bottom: 0px;
	font-weight: 600;
}

user-sync-custom.js

(function() {

	if ( ! wpus_ajax_settings.queues.length ) return;

	var queue_percentage = 100 / wpus_ajax_settings.queues.length;

	function request( index ) {

		show_logs();

		index = index || 0;

		if ( ! wpus_ajax_settings.queues[index] ) return;

		jQuery.ajax({

			url: wpus_ajax_settings.ajax_url,

			type: 'GET',

			data: wpus_ajax_settings.queues[index],

			success: function(data, textStatus, xhr) {
				request( ++index );
				increase_statistic( index * queue_percentage );
			}

		});

	}

	function show_logs() {

		if ( !! jQuery('body').find('.usc--status').length ) return;

		var markups = '<div class="usc--status">'+
			'<div class="usc--area">'+
				'<div class="usc--wrapper">'+
					'<h3 class="usc--heading">Please Don\'t close this window!</h3>'+
					'<div class="usc--slider">'+
						'<div class="usc--slider-inner"></div>'+
						'<div class="usc--slider-percentage">0%</div>'+
					'</div>'+
					'<p class="usc--slider-status">User Sync Syncing...</p>'+
				'</div>'+
			'</div>'+
		'</div>';

		jQuery('body').append(markups);

	}

	function increase_statistic( percentage ) {

		jQuery('.usc--area .usc--slider-percentage').text( parseInt(percentage) + '%' );
		jQuery('.usc--area .usc--slider-inner').css('width', percentage + '%' );

		if ( percentage == 100 ) {
			jQuery('.usc--area').remove();
		}

	}

	request();
	
})();

user-sync-custom.php

<?php

/**
* Plugin Name: UserSync Custom Sync
* Plugin URI: https://premium.wpmudev.org/
* Description: This plugin implemented chunk syncing, so syncing not get blocked in low execution time, only implemented user delete module
* Author: Ariful Islam @ WPMUDEV
* Author URI: https://premium.wpmudev.org/profile/itsarifulislam
* License: GPLv2 or later
*/

if ( ! defined( 'ABSPATH' ) ) {
    exit;
}


if ( ! class_exists( 'UserSyncCustomSync' ) ) {

    class UserSyncCustomSync {

        private static $_instance = null;

        public $user_sync = null;
        
        public static function get_instance() {

            if ( is_null( self::$_instance ) ) {
                self::$_instance = new UserSyncCustomSync();
            }

            return self::$_instance;

        }

        private function __construct() {

            global $user_sync;

            $this->user_sync = $user_sync;

            remove_action( 'delete_user', array( $this->user_sync, 'user_delete_data' ), 20 );

            add_action( 'delete_user', array( $this, 'user_delete_data' ), 20, 1 );

            add_action( 'admin_enqueue_scripts', array( $this, 'admin_enqueue_scripts' ) );

            add_action( 'wp_ajax_delete_user_sync_users', array( $this, 'delete_user_sync_users' ) );

        }

        function plugins_loaded() {

        }

        function admin_enqueue_scripts() {

            $queues     = array();
            $status     = get_transient( 'user_sync_status' );
            $action     = get_transient( 'user_sync_action' );
            $users      = get_transient( 'user_sync_s_users' );
            $sub_urls   = $this->user_sync->options['sub_urls'];

            if ( $status === false || $action === false || $sub_urls === false || $users === false ) return;

            if ( ! empty( $users ) && ! empty( $sub_urls ) ) {
                foreach ($users as $user) {
                    foreach ($sub_urls as $sub_url) {
                        array_push( $queues , array(
                            'site'      => $sub_url,
                            'user'      => $user,
                            'action'    => $action
                        ));
                    }
                }
            }

            if ( empty( $queues ) ) return;

            wp_enqueue_style( 'wp-user-sync-custom', plugins_url( '/user-sync-custom.css' , __FILE__ ) );

            wp_enqueue_script( 'wp-user-sync-custom', plugins_url( '/user-sync-custom.js' , __FILE__ ), array( 'jquery' ) );

            wp_localize_script( 'wp-user-sync-custom', 'wpus_ajax_settings', array(
                'ajax_url' => admin_url('admin-ajax.php'),
                'queues'   => $queues
            ));

            delete_transient( 'user_sync_action' );
            delete_transient( 'user_sync_status' );
            delete_transient( 'user_sync_s_users' );

        }

        function get_user_data( $user_id ) {

            global $wpdb;

            $data = get_userdata( $user_id );

            if ( !empty( $data->data ) )
                $user_data = (array) $data->data;
            else
                $user_data = (array) $data;

            $user_meta = get_user_meta( $user_id );

            $keys = array();
            // replace empty array on empty string
            foreach ( $user_meta as $key => $value ) {
                $keys[] = $key;
            }

            foreach ( $keys as $key ) {
                $user_meta[$key] = get_user_meta( $user_id, $key, true );
            }

            $prefix_fix = array('capabilities', 'user_level');
            foreach ($prefix_fix as $key) {
                if(isset($user_meta[$wpdb->get_blog_prefix() . $key])) {
                    $user_meta['wp_' . $key] = $user_meta[$wpdb->get_blog_prefix() . $key];
                    if($wpdb->get_blog_prefix() != 'wp_')
                        unset($user_meta[$wpdb->get_blog_prefix() . $key]);
                }
            }

            return array_merge( $user_data, $user_meta );

        }

        function user_delete_data( $userID ) {
            
            $user_data = $this->get_user_data( $userID );

            $status = $this->user_sync->options['status'];

            if ( "sub" == $status ) {
                //Adding login of user to list of deleted users on Sub site
                $deleted_users = (array) $this->user_sync->options['deleted_users'];

                if ( false === array_search( $user_data['user_login'], $deleted_users ) ) {
                    $deleted_users[] = $user_data['user_login'];
                    $this->user_sync->set_options( 'deleted_users', $deleted_users );
                }

            } elseif ( "central" == $status ) {

                $key      = $this->user_sync->options['key'];
                $sub_urls = $this->user_sync->options['sub_urls'];

                // Return if no sub_urls
                if ( false === $sub_urls ) return;

                set_transient( 'user_sync_status', 'syncing' );
                set_transient( 'user_sync_action', 'delete_user_sync_users' );

                $users = get_transient( 'user_sync_s_users' );

                if ( $users === false ) {
                    $users = array( $user_data['user_login'] );
                } else {
                    array_push( $users, $user_data['user_login'] );
                }

                set_transient( 'user_sync_s_users', $users );

            }

        }

        function delete_user_sync_users() {

            $user_login = $_GET['user'];
            $site       = $_GET['site'];

            $key        = $this->user_sync->options['key'];
            $sub_urls   = $this->user_sync->options['sub_urls'];

            // Checking key of security from Subsite
            if ( $this->user_sync->check_key( $site['url'], $key ) ) {

                $p      = array( 'user_login' => $user_login, 'overwrite_user' => $site['param']['overwrite_user'] );
                $p      = base64_encode( serialize ( $p ) );
                $hash   = md5( $p . $key );


                $this->user_sync->send_request( $site['url'], "user_sync_action=sync_user_delete&hash=". $hash . "&p=" . $p );

                //Update last Sync date
                $sub_urls = $this->user_sync->options['sub_urls'];
                $array_id = $this->user_sync->get_index_by_url( $site['url'], $sub_urls );
                $sub_urls[$array_id]['last_sync'] = date( "m.d.y G:i:s" );
                $this->user_sync->set_options( 'sub_urls', $sub_urls );
            }

            wp_die();

        }

    }

    function UserSyncCustomSyncRender() {

        $GLOBALS['UserSyncCustomSync'] = UserSyncCustomSync::get_instance();

    }

}

add_action( 'plugins_loaded', 'UserSyncCustomSyncRender' );