PHPExcel very slow – ways to improve?-open source projects PHPOffice/PHPExcel
I am in no means an expert in using PHPExcel, but the OfficeOpenXML format (the format of *.xlsx files) is itself a group of XML files packed in ZIP archive with *.xlsx extension. If you value your performance and know what kind of data you will be passing, maybe it is a better idea to build own XLSX generator, stripped down to the most important functions, maybe making some calculations on database layer etc. instead of parsing the whole document.
To do it, you can begin with analyzing files generated using smaller data sets (by changing extension from *.xlsx into *.zip, unpacking it and browsing through the contents of the single files). That way you could determine what you really need and generate it yourself (by creating appropriate XML files and packing them into ZIP archive, then renaming to have *.xlsx extension).
There is also specification of OfficeOpenXML, which is large (a couple thousands of pages), thus I do not propose reading it unless you really want to. Creating files to match the way they were generated by PHPExcel should be enough.
The solution mentioned above does not include any PHPExcel-related tips, because I am not an expert in it. I have been previously interested in OOXML standarization process however, and would be happy if knowledge about this standard would help you solve your problem.