I've used DB_File rather than BerkeleyDB::Hash, but I assume that the options available are similar. They are mentioned for the different DB types here as a part of the DB_File docs. I would assume that you are using what DB_File refers to as a DB_File::HASHINFO. The parameters you probably need to consider varying are the cachesize, bsize & ffactor.

However, the DB_File docs gives no information on how to vary these options for performance. Eg. An ever bigger cache does not always render better performance.

Optimising the options requires a fairly keen understanding of the nature of your data and the usage patterns of your application.

I did find that the documentation here, particularly section 2, was useful, but be prepared for doing a lot of experimentation.

The best guide I found to performance tuning was this page. Unfortunately, much of the advice relies upon your having access to one or more of the Berkeley DB utilities, which I never located for Win32. None the less, the information on that page proved very useful as a guide to some trial & error testing.