Problems are often caused by Excel users formatting more cells than they need. A useful workaround for this is the Excel format cleaner which is a free add in from Microsoft.
The typical example is highlighting the entire row to format instead of just the affected cells. Now that Excel has over a million rows and 16 000 columns, this method of formatting results in file size increases as Excel now has to remember the format of thousands, if not millions, of cells.
This is where an Excel format cleaner can assist in reducing the file size and increasing performance.