Transient Cache for Expensive Queries

/**
 * Get featured products with transient caching.
 *
 * @param int $count Number of products to return.
 * @return array
 */
function mytheme_get_featured_products( int $count = 6 ): array {
    $cache_key = 'mytheme_featured_products_' . $count;
    $products  = get_transient( $cache_key );

    if ( false !== $products ) {
        return $products;
    }

    $query = new WP_Query( array(
        'post_type'      => 'product',
        'posts_per_page' => $count,
        'meta_key'       => '_featured',
        'meta_value'     => 'yes',
        'no_found_rows'  => true,
        'fields'         => 'ids',
    ) );

    $products = $query->posts;
    set_transient( $cache_key, $products, 12 * HOUR_IN_SECONDS );

    return $products;
}

/**
 * Flush the cache when a product is saved.
 */
add_action( 'save_post_product', function( int $post_id ): void {
    // Delete all cached variations.
    foreach ( array( 3, 6, 9, 12 ) as $count ) {
        delete_transient( 'mytheme_featured_products_' . $count );
    }
} );