Back in May 2006 we posted some authentication rate numbers for Symas CDS 3.3.2 (based on OpenLDAP 2.3.21.) http://connexitor.com/blog/pivot/entry.php?id=49
On a 1 million entry database, we saw about 13K auths/second on a dual processor 2.8GHz Opteron system.
We've recently re-run the tests using OpenLDAP 2.4.5 on a dual processor 2.8GHz dual-core Opteron system (so, 4 cores now instead of just 2) and gotten 38K auths/second. One interesting thing to note is that we reached this same rate on back-bdb, with the slapd server at 100% CPU utilization, as well as with back-null, where slapd was only at 70% CPU. (This implies to me that there may have been a network bottleneck here, because we ought to have been able to drive slapd into 100% CPU usage even with back-null. Even gigabit ethernet has its limits...)
But the other nice point about this result is that while the number of CPU cores was increased by a factor of 2, the overall auth rates increased by a factor of 2.7. So we can see that OpenLDAP 2.4 is at least 35% faster than OpenLDAP 2.3. It also shows that the lightweight dispatcher is still scaling well with available CPU power.
Another point is the same thing I've harped on before - SLAMD is too inefficient on clients. We measured the 38K/sec rate using the slapd-auth C client that I just added to CVS HEAD. Using the SLAMD java client we could only reach 29K/sec, even with 12 client machines and 10 threads per client. Someone's going to have to sit down and write a C client that speaks the SLAMD server protocol and just fires off compiled C clients...