in addition to regular housekeeping of drawings (audit, purge, etc)
this page on Poor performance when working in Civil 3D https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Improving-Civil-3D-performance-s.html may assist in issues with performance in Civil 3D
also attached here is the Civil Site Design White Paper - Performance Settings.pdf on performance settings related to CSD that may go some way toward overcoming the bottleneck you are experiencing on your pc.
should the above not assist in easing the useage of the project, please provide the project files in a new support ticket
