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>I have tried the XLSXWorkbook class. It opens a small XLSX file quickly. But one with 20,000 rows takes a very long time but perhaps I do something wrong (although I only change the filename ...?)

Check if a SAX or document parser is reading/parsing. SAX usually faster for large files.

>Anyway, I found another tool which seems very powerful - https://www.coolutils.com/TotalExcelConverter - very fast conversion.

Wow, seems this could even handle .wq1 - my very old quattro sheets!