Comrade on "Left join slows down mysql query"

Hi everyone.

I am using an ecommerce plugin with my WordPress installation to sell products. By default this plugin orders my product post type just like WordPress orders the default post type--newest to oldest.

In my online catalog I have products that are in stock, out of stock (which can be re-ordered), and sold (which cannot be re-ordered). I need to order these products like so: in stock > out of stock > sold. To do this, I have created a meta key called "product_stock" whose value is numeric--3 for in stock, 2 for out of stock, 1 for sold. I order them by this value using the following two functions:

// Join for ordering products and search pages by product quantity
add_filter('posts_join', 'meta_join');
function meta_join($join) {
	if ( is_store_page() || is_search() ) :
		global $wpdb;
		//join stock
		return $join . " LEFT JOIN (
		SELECT *
		FROM $wpdb->postmeta
		WHERE meta_key = 'product_stock') AS stock
		ON $wpdb->posts.ID = stock.post_id";
	endif;
	return $join;
}

//Orderby for ordering products and search pages by quantity
add_filter('posts_orderby', 'product_meta_orderby');
function product_meta_orderby($orderby) {
	if ( is_store_page() || is_search() ) :
		global $wpdb;
		return " wp_posts.post_type DESC, stock.meta_value DESC, wp_posts.post_date DESC ";
	endif;
	return $orderby;
}

(As a side note, the is_store_page() function is one that I have created to detect whether my users are looking at one of my store pages or at a normal page or blog page.)

So basically I am wondering if there might be a faster way to implement the ordering that I need, because currently queries on my store pages run very, very slowly.