Drupal commerce and big cart
This is too vague to really answer well. A View may be slow for any number of reasons, but at a minimum, this View is loading 200 entities and potentially 400 if you're joining to products (or any other entity). You're going to need some pretty aggressive query caching, both at the database level and likely at the Views level, to perform at that scale of carts without having to change the way Drupal Commerce loads / manages cart orders.