the adoption of caching mechanism (entry cache, DB cache, filesystem cache) may generally give very good results; so if you have a 'relatively' small DB you could load it entirely in cache.
But more in general LDAP tuning is a too wide subject to be covered in a single forum thread due to the many implication and variables (i.e.: usage patterns, read/write operations, network infrastructure, hardware sizing, storage I/O subsystem, etc...)
I suggest you to start from the standard product documentation:
and in particular the "Installation and Tuning Guide".
P.S. 5.2 is a 'glorious', but still VERY OLD release... if possible, try moving to a more recent release!