OpenLDAP 2.3 has been unrivaled as the fastest directory server in the world for the past two+ years, but today that's no longer true. Now OpenLDAP 2.4 takes over as the most scalable, most reliable, highest performing directory server.
Thanks to our friends on the Samba Project and at AMD, we've been able to do some new benchmarks on an AMD quad-processor server. We tested OpenLDAP (2.4.5 and 2.4.6) on Linux as well as Microsoft's AD offerings on Windows 2003, and the results are now summarized on the Connexitor blog.
(The AD numbers aren't all in yet, we're still waiting for the directory import to finish.)