What version of Audit Trail are you using?  We added some indexes to the tables starting in 9.0 that should make queries more efficient.  Even if you are using the most recent version, you might benefit from adding indexes specific to the queries you commonly run.  The auditing database is pretty simple so there's a good chance you can locate the table columns based on the Audit Trail UI, and then add indexes to the ones you are filtering on.  Alternatively, if you have access to SQL Tuning Advisor it can suggest indexes to add based on your exact queries (which you can grab with SQL Profiler).  If there is a DBA available to help out this is probably something they've done before.

If you are using AT 8.x and aren't in a position to upgrade the whole system to 9.x, you might consider upgrading just the Audit Trail portion (which will give you all of the indexes we've added).  Normally you can't upgrade clients to a higher version than the server, but Audit Trail is (currently) an exception since it just requests audit logs.  The only confusing thing is that new events types (like Business Processes) show up in the reporting/filtering UI.

As far as a guide to integrating with BO, you'll need this information to decode the event type and there are predefined views created for the different event classes.