Unfortunately that is the nature of the beast with large video files that initially have little to no tag data. As soon as that header needs to be increased, the entire file gets rewritten. If you update several large files like this at once, you could conceivably be tying up some system resources for quite a while.
As far as the request for a sysem sound after the save completes, were you able to get that enabled? You should be able to enable that in Options>Messages preference.
