Speed Up Large WordPress & WooCommerce Website Back-End

functions.php

<?php
add_filter( 'postmeta_form_limit', 'limit_postmeta_keys_to_zero', 10, 1 );
function limit_postmeta_keys_to_zero( $limit ) {
	global $post;
	// Only for these post types
	$valid_post_types = array(
		// WooCommerce Subscriptions
		'shop_subscription',
		// WooCommerce
		'shop_order',
		'shop_coupon',
		// Sensei
		'course',
		'lesson',
		// WordPress
		'page',
		'post'
	);

	if( is_admin() ):
		if( in_array( $post->post_type, $valid_post_types ) ) {
			return 0;
		}
	endif;

	return $limit;
}