When loading the outbox of a user, all the public posts of that person are loaded into the cache, but eventually only the first and last post are used to determine the max_id and since_id, and the count (to determine totalItems). This results in the request taking a lot of time and loading all posts into memory (I've seen 8 seconds loading time and even 500's for a user with 5000 posts)

Example: Getting @Gargron's outbox at https://mastodon.social/api/activitypub/users/1/outbox just returns a 500.)

• I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• This bug happens on a tagged release and not on master (If you're a user, don't worry about this).