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

Greg will give you better insight, but I think the bottleneck lies in the XML parser that must load what is a very long document and build an XML tree before going into its processing.