In many large models, most of the geometry consists of objects that can be represented as an extrusion of a planar curve.
Rhino 5.0 has a new lightweight extrusion object that uses less memory, meshes faster, and saves smaller than the traditional polysurface used in Rhino 4.0. In Rhino 5.0 WIPs released on or after October 4, 2010, commands like Box and Cylinder create lightweight extrusion objects. Other commands like Pipe and ExtrudeCrv make lightweight extrusion objects when possible and make traditional polysurface objects when the resulting geometry is too wiggly to be handled by a lightweight extrusion object.
Customers were creating architectural models with tens of thousands of objects that were simple extrusions. When traditional polysurfaces were used to represent the objects, the models were sluggish and used lots of memory resources. When these models use the light-weight extrusions, the models were responsive and there was plenty of memory available.
The new UseExtrusions command can be used to control the use of lightweight extrusion objects. If you want commands like Box, Cylinder, Pipe and ExtrudeCrv to make lightweight extrusion objects, then run UseExtrusions and select the “Extrusions” option. If you want these commands to make traditional polysurfaces, then select the “Polysurface” option.