This is actually a bug of IPS UTF8 Database Converter (by @Matt ?). I tried converting my database on 3.4.7 to UTF-8 using the latest version of the converter, and got the same problem. There's nothing to do with 4.0. This is a serious problem for me
I'm having two versions of the community running in parallel, on the same server. The 4.0RC3 one is an upgrade from the backup of my live board, does not have any visitor except me, and still is very slow compared to the other.