Custom fields are stored as meta data in a separate table which are separate to post data and that’s why you don’t get it with wp_query.
By default, WP_Query returns the standard WP_Post objects for the posts being queried. I believe with some clever rewrite and use of the filters given in WP_Query you can add objects to the returned WP_Post objects array.
In my opinion, it will hurt performance more as you will need to join results in your query as custom fields are not saved in the wp_posts table, but in the wp_postmeta table.
You can loop through the posts and attach the required meta data yourself. with $posts->posts; in your example
Retrieving post meta is really fast and it does not require any extra instance of WP_Query. You can simply call the custom field with get_post_meta(). WordPress was very thoughtful when the custom fields was introduced. They added a cache to cache them, so whether you are querying 1 or 100 custom fields, you are hitting the database once, superfast.
In my opinion, the extra database call and the actual time spent is worth it and faster than rewriting WP_Query in such a way to include custom fields in the standard post object returned by $posts.
HTH
