Add all post meta using single Insert SQL for migration.

fast_post_meta.php

<?php
/**
 * $arr all meta in associative array.
 * $post_id where to insert post.
 **/
function add_post_meta_all( $arr, $post_id ) {
			global $wpdb;
			$inserts = array();
			foreach ( $arr as $key => $val ) {
				if ( is_array( $val ) ) {
					foreach ( $val as $item ) {
						// insert multiple post meta if it is array
						$meta_value = wp_unslash($item);
						$meta_value = sanitize_meta( $key, $meta_value, 'post' );
						$inserts[] = $wpdb->prepare( "(%d,%s,%s)",$post_id,$key,$meta_value);
					}
				} else {
					$meta_value = wp_unslash($val);
					$meta_value = sanitize_meta( $key, $meta_value, 'post' );
					$inserts[] = $wpdb->prepare( "(%d,%s,%s)",$post_id,$key,$meta_value);
				}
			}
			if ( ! empty( $inserts ) ) {
				$sql            = "INSERT INTO {$wpdb->postmeta} (`post_id`,`meta_key`,`meta_value`) VALUES " . implode( ', ', $inserts );
				$maybe_inserted = $wpdb->query( $sql );
				if ( ! $maybe_inserted ){
				  error_log( $sql . " error: " . $maybe_inserted );
					//\WP_CLI::error( $sql . " error: " . $maybe_inserted, false );
					return false;
				}
				return true;
			} else {
				return false;
			}
		}