To ensure that older workbooks are calculated correctly, Excel behaves differently when you first open an old workbook than when you open a workbook created in the current version.
When you open a workbook created in the current version, Excel recalculates only the formulas that depend on cells that have changed.
When you use open a workbook that was created in a earlier version of Excel, all of the formulas in the workbook — those that depend on cells that have changed and those that do not — are recalculated. This ensures that the workbook is fully optimized for the current Excel version.
Because complete recalculation can take longer than partial recalculation, opening a workbook that was not previously saved in the current Excel version can take longer than usual. Once you save the workbook in the current version, it will open faster.
