“Twitter Blog: The Replies Kerfuffle”

Even though only 3% of all Twitter accounts ever changed this setting away from the default, […] every time someone wrote a reply Twitter had to check and see what each of their followers’ reply setting was and then manifest that tweet accordingly in their timeline—this was the most expensive work the database was doing […] Ideally, we would redesign and rebuild this feature but there was no time, hence the sudden deploy.

Well, okay then, fine. (Although, if only 3% of users changed the setting, you could easily enough create a dedicated database of those users on separate servers, and only check that and save yourself a lot of grief, couldn’t you? I mean, rather than promise an unwieldy amount of customization in the future, and hope people forget.)