Type post field + clone + 2k posts

yes, autocomplete, than typing 2-3 letters. Also you first thouth right - if you will make request via ajax to standart query for all posts - it will not reduce the number of requests, but it will improve the page loading time with requesting heavy query async. Also for now, I have reduced my admin page load time from 30s to 6s (it's localhost, prod server more better) with simple construction:

		$all_products = [];

		$args = array(
					'posts_per_page' => -1,
					'post_type'      => 'product'
		);

		$query = new WP_Query( $args );
		if ( $query->have_posts() ):
			while ( $query->have_posts() ): $query->the_post();
				$all_products[get_the_ID()] = get_the_title();
		 	endwhile;
		endif;

and then

array(
					 'id'           => 'order_products',
					 'type'         => 'group',
					 'clone'        => true,
					 'sort_clone'   => true,
					 'collapsible'	=> true,
					 'save_state'	  => true,
					 'default_state'=> 'collapsed',
					 'group_title'  => array( 'field' => 'product_id' ),
					 'add_button'   => 'Добавить товар',
					 'fields'       => array(
							array(
									'name'        => 'Товар',
									'id' 	        => 'product_id',
									'type'  			=> 'select_advanced',
									'options' 		=> $all_products,
									'class'				=> 'product_id'
							),