Since this is not a Mendix specific question, you can find similar discussions on stack overflow. Perhaps this one can help
https://stackoverflow.com/questions/19653063/apache-poi-taking-long-time-to-generate-excel-sheet
I have seen this happen when the file format is XLSX, did you try XLS? (i know the latter is an older version but worth a try and it anyways has good support and community still uses it)