I have received a few emails from people saying their new version of AutoCAD was slower at loading files and found a common situation. Opening older format DWG files can take more time but there could be a good explanation for this. If you are trying to open a file from a prior format, AutoCAD must convert to the new version first. Try saving in the current AutoCAD released format such as 2007 and then opening to compare opening time instead of for example trying to open a 2000 format DWG into AutoCAD 2007.