We built SongStitch because we wanted an open source[1] and performant way to generate Last.fm collages (a popular thing to do in the Scrobbling community). Written in go with performance in mind, it allows fast collage generation up to 15x15 grid size — though we could go larger if we run the software on less-modest fly.io machines. Images can also be embedded such that they're always up to date when loaded.
1. https://github.com/SongStitch/song-stitch