Is it true that this may also occur on Flac files the first time it is added? Possibly because it needs to rewrite the entire file (Tag info is at the beginning).
I seem to recall someone telling me this...... and I know for a fact that MP3s update cover art for me quickly, while I see the same performance as you when updating cover art stored inside of FLAC files. So either there is some truth there......or my virus program exclude rules are not doing what I think,.