A couple new results for back-mdb as of today.
first second slapd sizeback-hdb, 10K cache 3m6.906s 1m39.835s 7.3GB back-hdb, 5M cache 3m12.596s 0m10.984s 46.8GB back-mdb 0m19.420s 0m16.625s 7.0GB
back-mdb 0m15.041s 0m12.356s 7.8GB
Next, the time to execute multiple instances of this search was measured, using 2, 4, 8, and 16 ldapsearch instances running concurrently. average result time 2 4 8 16 back-hdb, 5M 0m14.147s 0m17.384s 0m45.665s 17m15.114s back-mdb 0m16.701s 0m16.688s 0m16.621 0m16.955s
back-mdb 0m12.009s 0m11.978s 0m12.048s 0m12.506s
So back-mdb is faster than back-hdb whenever there's more than one query running. Also with result times of 0m11.699s measured, back-mdb is within 7% of back-hdb's speed even in the single-query case, where hdb has zero lock contention and all it has to do is dump cached entries from RAM (i.e., back-hdb is doing practically zero work at all).