There can be different reasons for slow performance when copying and pasting objects in drawings.
The most common cause are large numbers of layer filters. One easy way to check and remove these is the FILTERS command that is described here http://usa.autodesk.com/getdoc/id=TS1056902
If you are using AutoCAD 2012, please also ensure that SP1 is installed.
If Layer filters can be ruled out, check if the drawing contains a large number of annotation scales. More information and tips how to remove those can be found here: http://usa.autodesk.com/getdoc/id=TS10031804