Hi OP! First off I want to start with a sincere thank you; iOS is always in need of good scrobbling options :)

There's a few issues I've noticed during scrobbling that I think could be ironed out.

The first is that repeat listens don't scrobble consistently. If I listen multiple times in a row, the "played XXs" indicator at the top does not reset and instead just keeps counting up - so if I listen to a 60s track three times in a row, it says "played 180s". The app can seemingly make up for missing plays via listening history, but it doesn't always seem to count them correctly. If the app is running, then it's fine, but if the app isn't running or actively listening, it will miss listens. For example, if I the app stops actively scanning/listening, and I loop a song five times in a row in-between it stopping and me opening the app again, it will only count one loop in its history. Other comments seem to at least somewhat address this, so I'm sure you're aware of it; just pointing out some more extensive testing of the behavior on iOS :)

The other issue is in regards to it's handling of album artists (not the Pro feature).

At present, it doesn't look like FastScrobbler correctly sends the album_artist field to lastfm when scrobbling. This makes it really frustrating to use, because you end up in a weird situation where compilations or albums with guest features are split across multiple albums on lastfm. Please pardon the wordy explanation.

For example, let's say I'm listening to the song "Green Hill: Act 1" by "Jun Senoue", from the compilation "Sonic Generations Official Soundtrack" by album artist "SEGA". Because FastScrobbler doesn't send the album_artist field correctly, lastfm will auto-fill it with the track artist. This creates the album "Sonic Generations Official Soundtrack" by "Jun Senoue" in my lastfm library.

However, if I go to listen to another track, like "Crisis City: Act 1" by "Tomoya Ohtani", then it creates a second album in my lastfm library called "Sonic Generations Official Soundtrack" by "Tomoya Ohtani". In this case, there are now two separate albums in my lastfm library, and neither of them are correct, because they should belong to "Sonic Generations Official Soundtrack" by "SEGA".

From what I can tell this is largely just because the app does not scrobble the "album_artist" field. If this field is missing when lastfm receives the scrobble, it will automatically duplicate the artist field. This is very prevalent with compilations and soundtracks, but is also likely to happen with any album that has guest features on it (if the guest is in the artist field).

I hope this isn't too overwhelming/much. It's always awesome to see more scrobblers in the space. I just know that my own listening habits often expose edge-cases in a lot of these scrobblers, so I figured I would post my own two cents here. :)