I would say that the attribute data could be very important in some instances. Would it be possible to have a toggle where the metadata is ignored for better performance or included for greater relevancy with slower performance.
The other idea that comes to mind is to allow the creator to decide at the time of upload whether or not to include the metadata.
That being said, if those aren't possible, I don't think much more than a 10-15% slowdown would be considered acceptable. Since the original DWF could always be made available along with Design Review -- of course that is for Windows only -- that data can be accessed.