If we have a cache file to store the precomputed hashes then the amount of time taken for each individual hash isn't so important. Also I don't know of many people that upload that much music in one go
It's only the initial hashing that is the time consuming part. Once that's done then it should in theory be reasonably quick.
One question though, how are you going to implement this for players that already have music on? You'd have to download the file, hash it, delete the downloaded copy and finally upload the new tag back to the player.
- Trevor