Jade

On desktop? Yes. On Element X? Not yet. Requires additional setup, as it’s not a core part of the homeserver

Generally you can’t tag stuff from your phone. Either tag it on MusicBrainz.org / ListenBrainz.org and then pull that in via Picard or Beets or make playlists

A lot of the people reporting issues with Matrix being slow and resource heavy are reporting issues with Synapse, which is Element’s big Python implementation. My Continuwuity instance, which is a server written in Rust, uses a fraction of a CPU and a total storage in the hundreds of megabytes. A few less features, but it has most of the ones people care about.