I have a similar issue. It seams like the problem started, when Plex switched the music agent to Musicbrainz.org. If the album/track isn’t found in MB (musicbrainz), it looks like (from the logs), that Plex is doing string matching per track title (levenshtein distance I guess). This results in excessive CPU and memory consumption.
I have 98 albums which can’t be matched. So adding a single album/track, results in PMS running 80-97% CPU for +20 minutes (for the 98 unmatched also - everytime). In the end, I decided to disable “Include music libraries in automatic updates” (under Settings > Library). Which means, I can add multiple albums/tracks and update manually.
