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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.
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>Just made an experiment with a 10MB/160k rows XLSX file. It is ingested by an XLSX reader class of mine that rely heavily on the XML parser to hold and process data (there is only a cursor for shared strings, that is, the strings that are used in the spreadsheet) in 14 seconds. The same file, imported by LibreOffice Calc, takes 12 seconds to load (of course, LO Calc has to transform the file into ODF format, in the process).
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>Although these are not actual figures for XLSXWorkbook, I believe theirs should not be too far away. In perspective, such timings seem acceptable to me.

I tried XLSXWorkbook - takes far, far longer than that. In fact, I just killed VFP - I could see this was not going to be a realistic solution for my needs. Total Excel Converter from CoolUtils converts in 2 seconds or less (20k rows x 7 cols).