Increasing performance of an XLSX import feature

In a Symfony2 application we’re developing for a client, a key feature is the import of some XLSX files containing previous representation of the data (let’s call it legacy data) into the system.

When the team started to work on this feature the client provided us with test files to analyze the structure, the data that needed to be imported, etc. This files were rather small, only a few hounders of kilobytes (almost all of the files were under 500Kb). So one of the developers in our team build the feature to import the data using the amazing PHPExcel library. And we all were happy for some time.