>Each reply is a file in a thread folder in a board folder in the database folder.

That's a pretty good approach actually, if it was one file per thread instead. And even better if it was one file per board/site without any of the new database cancer, and just kept the entire thing in memory for most of the work duration. Could use per thread compression for bonus points. Many site hosters seems to be cucked in the file department, they seem to force you to store files in a database, which I think is absurd.

It could sync memory content with a file once a hour or so.