I was previously on 2000 and upgraded to 2004,I noticed a difference to.Must be due to the extra little tweaks and features theyve added.2004 is hungrier on resources,so I guess 2005 will be a little hungrier stil.You could try freeing up some system resources via the MSCONFIG,disabling programs that are running in the background but arent actually required.The transparency feature for the tool palettes can also slow the program up a bit.Found this of the net it maybe of some help:
The layer command is slow and sheetsets and fields are unable to be created. The old LT versions were basically 3d disabled, now the drafting features are being removed/disabled as well. If your going to spend the money get autocad full or look for another product, we are very unhappy with this change, especially how it was not very well publicized and we didn't find out until we purchased enough for multiple offices and had it installed.