I'm generating .xls (not .xlsx) files, using a third-party XLS library. For the most part, it works great.
One problem is that it operates, roughly, by building the entire file (e.g., in memory, though on disk wouldn't be much better), and then sending it off. For large files, simply having this many bits can hurt!
OTOH, when we want to send CSV, we can stream directly from the database to the client, which is really fast because we never need to have or know the whole CSV file -- it only really exists once it gets downloaded to the client.
Is it possible to stream XLS? I think the situation may well be that no XLS library supports that, but at this point I'm just wondering if it's possible. If it is, I may jump in and try writing/modifying one to do that.