Civil 3D Performance Issues

If your Civil 3D software is experiencing major performance degradation where files take too long to open (depending on complexity), plotting is slow, purging is slow, and AUDIT does nothing to help alleviate the issue, then you may want to attempt the recommendations within this Autodesk document:

Civil 3D Performance Degradation

Please note that the downloadable zip file mentioned in the solution is at the bottom of the page.

And one last very important note:  The update needs to be implemented on every user’s machine. The performance problem will return if an unpatched version of Civil 3D is used to open a file. The performance problem is fixed at the time a file is saved in the updated version of Civil 3D. The first time a user opens a file the update will not be evident. Performance will greatly improve after the first save. External references will need to be resaved to see the benefit of the update. 

Check this to verify if your software needs the new DBX file: