Picard always seems to redownload all cover art, irrespective of whether cover art has actually changed. This significantly slows down the process of updating tags of previously tagged collections, and also causes unnecessary stress on the server hosting the cover art.
My suggestion would be to store a hash of the cover art in the tags and only download new art when the hash has changed. This would greatly increase the speed of updating existing collections. Would something to that extent be possible?
While there is no such mechanism for now (CAA doesn’t provide any hash, but it provides edit number that could serve to detect changes), it should be noted Picard is using a disk cache, and doesn’t download images if they were recently downloaded.
Look for (CACHED) in debug log:
D: 09:51:23,892 picard/webservice/__init__._handle_reply:415: Received reply for http://ia800701.us.archive.org:80/0/items/mbid-e8d24388-5176-40a7-812a-89308afb24b9/mbid-e8d24388-5176-40a7-812a-89308afb24b9-21655633641_thumb500.jpg: HTTP 200 (OK) (CACHED)
Of course there is room for improvements, we could check if index.json was modified to start with, and don’t even try to download images, etc… Patches are welcome.