I know the recommendation is not to export large of amounts of detailed data. However, some of our internal groups want this ability and do not mind waiting.
One thing I have noticed is that trying to export 10k rows or more of more than a few columns causes apache threads to consume well over 600MB of memory. I've increased both the timeout and memory parameters for PHP to accomodate this.
However, with multiple concurrent users, our server's memory can easily come under great pressure with concurrent exports.
I realize the memory is likely an issue within PHPExcel. Have you tested any of the other PHPExcel_CachedObjectStorageFactory settings?
I'd be happy to test locally if it's a user-configurable.
Also, is there any chance of using something else to generate csv output besides PHPExcel given its performance issues?