Why do comments from lemmy.world users not appear until 4 days later?

It's a recurring issue and has been for weeks now. The most recent one I noticed was this comment, which despite being tagged 4 days ago, only just recently appeared in my inbox.

A while ago it was only 2 days, but the problem seems to be getting worse and worse. In a meta post on my instance, an admin noted that they don't believe it's anything they can resolve on their end, so I'm posting here on the assumption that it must be a LW issue.