Alrighty, so here's the deal. The original version of the download code used an out of date, unmaintained spreadsheet library that wasn't compatible with the version of PHP we use. Naturally I couldn't sleep at night knowing that there was a fresher, newer, hipper library out there, so I rewrote everything to use the new one. Unfortunately, the new, sexy library uses around 6x as much memory, and takes roughly 50x longer to generate the data. So, I've switched back to the humble library. Charlie Brown would have wanted it that way. I've tested the code with Wajo's mega-huge collection, and it seems to work fine.
I was careful to port the bug fixes I did back to the original code, but please let me know if you have any problems.