Unindexed search times for DSEE 6.3.1 versus DS 5.2 - 5.2 wins?!!?

Question

We are running 6.3.1 and 5.2 systems in parallel. On both systems, the displayname attribute is not indexed. Upon issuing an ldapsearch on filter (displayname=Mark Markie-Mark) for example, the 6.3.1 system will take about 90 seconds to complete the query. On the otherhand, the same query on the 5.2 system takes about 10 seconds. These queries were done by BINDing as a standard user over SSL.
When I use directory manager account, the queries both return in the same amount of time.
Indexing the displayname attribute for equality fixes the duration drastically (within a second results are returned). However, why would the newer version of the software take longer on non-indexed searches such as this? Our new hardware is faster as well.

Answer 1

ACIs and number of entries are the same between the two systems. I used the dsmig command to migrate security, config, and schema settings. One thing I left out is that the 5.2 system and 6.3.1 system are part of the same replication topology, with 5.2 masters replicating to both 6.3.1 consumers and 5.2 consumers.
We also came to the conclusion that ACI evaluation could be the problem, but why would indexing the attribute affect the ACI evaluation performance? Has the ACI evaluation engine significantly changed between 5.2 and 6.3.1?