Howard Chu is the Chief Architect of OpenLDAP. He has some information on the emerging 2.4 release, and he states

I think it goes without saying that no other Directory Server in the world is this fast or this efficient.

Read it, and also look at his February 2007 presentation. Pages 25 ff. have some interesting numbers on them…

LDAP and Linux :: 17 Feb 2007 :: e-mail