Incremental Save Percentage or ISAVEPERCENT is a system variable that Autodesk introduced to improve the speed of the save command. It does this by not performing a full save every time you save the drawing. Changes and new objects are saved but if objects are deleted they are only marked as deleted in the drawing database rather than being completely removed. The Incremental Save Percentage sets the percentage of this potentially wasted space in a drawing file. By default it is set to 50, which means that 50% of the drawing file size could be wasted space.
The benefit of this is that saving the drawing is faster. However there are several significant problems created by this as well. You will have larger drawing files as a result of this wasted space. There are also increased instances of drawing corruption, problems with certain antivirus software, and the potential for incorrect save times to be shown on a drawing when viewed in Windows Explorer.
In my opinion the potential problems outweigh the benefit of faster save times. Particularly when you consider the increased performance of computer hardware I doubt that you would really notice that much of an improvement in save time any way. So my recommendation is to set your Incremental Save Percentage to Zero. You can find the setting on the “Open and Save” tab of the AutoCAD Options dialog box, or just type in ISAVEPERCENT at the command line. This will perform a full save each time you use the save command. Your drawing files will be smaller and you will avoid many potential problems.