The following information is straight from the Autodesk Knowledge Base about Civil 3D 2006. It offers suggestions on how to improve performance with Civil 3D 2006.
1. Disable the Rebuild Automatic option for Surfaces and Corridors.
2. Change styles to limit the objects displayed in the drawing.
3. Use data shortcuts to reduce the amount of data in a working drawing.
4. Copy project files located on a network drive to a local drive.
5. Use the Cache model tab and the all layout tabs option on the System tab in the Options dialog box.
6. Increase Autosave timings on the Open and Save tab in the Options dialog box.
7. Change the value of the INDEXCTL variable to 0 (zero).
8. Turn off the Dynamic Input methods and selection previews.
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