Speeds up the loading of /wp-admin/edit.php?post_type=shop_order and /wp-admin/edit.php?post_type=subscription.

OrdersList.php

<?php

namespace UniversalYums\Admin\Performance;

class OrdersList {

	/**
	 * The single instance of the class.
	 */
	protected static $instance;

	/**
	 * Instance.
	 */
	public static function instance() {
		if ( null === self::$instance ) {
			self::$instance = new self();
		}

		return self::$instance;
	}

	public function __construct() {
		add_action( 'posts_where', [ $this, 'change_where_clause_for_admin_order_list' ], 99, 2 );
	}

	/**
	 * Check if we are on the admin orders or subscriptions "All" page.
	 *
	 * @return bool
	 */
	public function is_admin_all_list_page() {
		if ( ! is_admin() ) {
			return false;
		}

		if ( wp_doing_ajax() ) {
			return false;
		}

		$request = wp_unslash( $_SERVER['REQUEST_URI'] );

		if ( strpos( $request, '/wp-admin/edit.php' ) === false ) {
			return false;
		}

		if ( ! isset( $_GET['post_type'] ) ) {
			return false;
		}

		if ( isset( $_GET['post_status'] ) ) {
			return false;
		}

		if ( 'shop_order' === $_GET['post_type'] || 'shop_subscription' === $_GET['post_type'] ) {
			return true;
		}

		return false;
	}

	/**
	 * If we are on the admin Orders or Subscriptions 'All' page, simplify the WHERE clause by removing post_status search.
	 *
	 * Potential Issue:
	 * We might see orders that don't have a WC status. But, maybe that's also a good thing?
	 *
	 * @param $where
	 * @param $wp_query
	 * @return mixed|string
	 */
	public function change_where_clause_for_admin_order_list( $where, $wp_query ) {
		global $wpdb;

		if ( ! $wp_query->is_main_query() ) {
			return $where;
		}

		if ( ! $this->is_admin_all_list_page() ) {
			return $where;
		}

		if ( 'shop_order' === $_GET['post_type'] ) {
			$where = "AND $wpdb->posts.post_type = 'shop_order'";
		}

		if ( 'shop_subscription' === $_GET['post_type'] ) {
			$where = "AND $wpdb->posts.post_type = 'shop_subscription'";
		}

		return $where;
	}
}