In Excel, if you have a big spreadsheet, it can start to get slow. Really slow. At that point, you can switch off the automatic calculation of formulas.
To do so, use the Tools | Options command, switch to the Calculate tab, then select Manual. (The keyboard method to navigate there is Alt+T for the Tools menu, O for the Options command, Ctrl+Tab to switch tabs to the Calculate tab, then Alt+M to select the Manual radio button. Then press Enter to close the dialog box.)
From now on, any time you change a number or formula, the spreadsheet won’t update itself until you press F9.