I've seen there's a 'checkpoint' option along with 'dbnosync' which can perhaps increase performance at the expense of reliability, but I wouldn't want to use that in production.
The db env flags for mdb can be important. I.e., I use writemap myself, and ldapadd is quite fast.
--Quanah -- Quanah Gibson-Mount Architect - Server Zimbra, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration