Every logged-in user gets a custom home page generated from their personal list of subscribed subreddits. They may be generating this page on a per-post basis rather than a per-sub basis, expecting that in the average case users aren't going to be subscribed to a bunch private subs that they have no access to.

If that is the case, then generating the home page for a user is going to be hitting a lot of posts from private subs which have to be skipped over until enough accessible posts are reached to fill the page. And in the case of the official app, infinite scrolling is going to lead to constant pressure to generate the list of posts.