The optimization process and recommendations:

1. Avoid Subqueries (query line: 27): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
3. Sort and Limit Before Joining (modified query below): In cases where the joins aren't filtering any rows, it's possible to sort and limit the amount of rows using a subquery in the FROM clause, before applying the joins to all other tables.

Optimal indexes for this query:

ALTER TABLE `categories` ADD INDEX `categories_idx_id` (`id`);
ALTER TABLE `category_user` ADD INDEX `category_user_idx_user_id` (`user_id`);
ALTER TABLE `files` ADD INDEX `files_idx_id` (`id`);
ALTER TABLE `locations` ADD INDEX `locations_idx_id` (`id`);
ALTER TABLE `products` ADD INDEX `products_idx_updated_at` (`updated_at`);

The optimized query:

SELECT
        products_id,
        userid,
        users_and_products.slug,
        users_and_products.city,
        products_updated_at,
        (SELECT
            GROUP_CONCAT(categories.name) 
        FROM
            categories 
        JOIN
            category_user 
                ON categories.id = category_user.category_id 
        WHERE
            category_user.user_id = userid) AS list 
    FROM
        (SELECT
            products.id AS products_id,
            products.updated_at AS products_updated_at,
            products.creator_id AS products_creator_id 
        FROM
            products 
        ORDER BY
            products.updated_at DESC LIMIT 50) AS products 
    JOIN
        (
            SELECT
                users.id AS userid,
                files.slug AS slug,
                locations.city 
            FROM
                users 
            LEFT JOIN
                files 
                    ON users.file_id = files.id 
            LEFT JOIN
                locations 
                    ON users.location_id = locations.id
            ) AS users_and_products 
                ON products.products_creator_id = users_and_products.userid LIMIT 50