Finally, one of the best performance tune-ups you can do is to make sure you are maintaining the right indices. Too few indices can lead to poor search performance. Too many indices can lead to poor update performance. For example, the LDBM backend would be perfectly happy to maintain substring and approximate indices for the objectclass attribute, but this would not be useful and would just slow down update operations. If your database has many entries and is handling queries for substring equality on the surname attribute, you should make sure to maintain a surname substring index so these queries are answered quickly.
So, take a look at the index lines in your slapd configuration file to ensure that only those indices that make sense and are needed are being maintained.