We put some top tuning tips in the DS 5.2 docs, http://docs.sun.com/source/816-6697-10/tips.html
That chapter is quite short and to the point.
Directory Server can be a disk intensive application, in particular if you have lots of updates. At that point, you want to keep logging and writes to the database from becoming a bottleneck. Some techniques for moving that bottleneck include distributing writes across disk subsystems, and turning off unnecessary logging.
Another thing you can do is limit the amount of resources clients can use. We talk about adjusting operational limits.
Hope this is of some help.