It may depend up on which version of AutoCAD and what are you are Resources like RAM & Processor Speed.., and Try to empty the temp folder.., and remove all the .bak files
.bak extension files are temporary data storing files whenever you open the dwg it will open a .bak file
try to empty them
check for any attack of malware or Virus on PC it may cause too .., and if you are using authorized autocad then you can contact autodesk forum directly post your Question
http://forums.autodesk.com
try to look at the following for further more info..,
http://forums.autodesk.com/t5/AutoCAD--DWG/Autocad-2012-amp-2013-sometimes-hang-very-slow/td-p/3530110
Cheers
You must possess4 GB RAMat the very least
System requirements for AutoCAD:
http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=&linkID=9240617