CRM Audit is as well optimised as it can be to avoid performance overhead when recording audit data. In my experience, it doesn't have any discernible impact on user-entry of data, though an overhead can be detected if bulk-loading data.
There's not much more that you can do other than test it, and monitor the performance with and without auditing
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk