Alias dereferencing and performance

Hi, I have a question about the alias dereferencing and mdb performance. I installed from source ldap 2.4.42 with mdb support. In our current directory we have 367 007 objects and 42262 aliases. During the test I observed that the MDB is slower than the HDB during search with aliases dereferencing. I performed test search in small scope (11115 objects where 6991 is aliases to other part of structure) with filter objectClass=User and dereferencing aliases set to always. I see that query to HDB take about 20s and query to MDB about 29s. On both server the load are low. Are in ldap configuration any option than can tune the alias dereferencing or we should redesign the structure to reduce number of aliases?

*Karol*