I use AutoCAD Mechanical. Here is a performance tip for modeling.  Depending on the size and complexity of the 3D drawing and your computer's performance, the visual style of each of the viewports can be set to wireframe. When doing this, the zoom and pan functions are much smoother, and it reduces the chance your computer will lock or freeze. In the Viewport Properties window, you can change the selected viewport's visual style, as well as the shade plot setting (which will take effect on paper). This way you can view in wireframe for smoother operation, and plot lines hidden, realistic, etc.

NOTES FROM CADALYST TIP PATROL:  Good performance tip that also applies to AutoCAD.  Using less-detailed visual styles while modeling will help the software and workstation work at a better performance level.  Good advice. Tested in AutoCAD 2009.