For archival purposes I am posting to let interested parties know that Spreadsheet::XLSX::Reader::LibXML v0.38.4 was posted to CPAN with caching for the shared strings file which appears to possibly be the worst culprit for speed hits on large files with lots of text. It is set to default on but can be turned back off with the attribute cache_positions. I continue to welcome any specific feedback or test files for optimization on github

And here is a link to the documentation for the implementation of the :just_the_data flag mentioned by MidLifeXis++