this doesn't make a lot of sense to me. do you have some archetectural insight, or just guessing?

fetching a list of default/subscribed subs and comparing it against the whole list of private subs every time someone loads your homepage sounds like a pretty bad design. why wouldn't something as fundamental as a private/public flag be stored in the main db entry for the subreddit?

either way, this doesn't seem like a probable root cause. checking a list of default subs against the list of private subs would be expensive under any circumstances. having all the default subs added to the private list wouldn't really change the already terrible performance by much. there just aren't that many default subs to begin with.

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.