You can create a local extract of your Excel Sheet by Right-Clicking on the Data Source, then choosing "Extract Data."
After you have created a local extract, right-click the data source, then Select "Extract -> Optimize." This should upload all of your calculated fields into the extract, which would allow you to query them directly instead of recalculating.
Does this help?
Thanks for the response. I currently am using an extract of the Excel file pulled from the database in the workbook with all of the calculations. As a result, it is already optimized and everything. The idea behind using two workbooks is to keep the workbook for presentations free from having to compute any complicated formulas. So in the example of ad serving I would have some kind of field called "Gross Media Cost" in the calculation workbook that takes the Net Media Cost and adds the Agency Fee, Ad Serving and other calculations to the figure. In the crosstab that would serve as the data source for the presentation workbook I would just include the "Gross Media Cost" column which was calculated in the calculation workbook and thus does not have to be calculated in the presentation workbook.