How to force Microsoft Excel (.xlsx) files to use only inline strings for cellvalues

Question

whenever I edit Microsoft Excel file *.xlsx manually, the cell values are stored as SharedString. How can I force editing cell values in .xlsx files to use only inline strings?
Or any compatible xls file formats which supports 400K rows with only inline strings?

Note: I am facing very bad performance issues reading .xlsx files(400K rows and 8 columns) with SharedString CellValues through OpenXML library. It is read faster if the excel file has only inline strings.