As of last major patch (24.04.2018) Performance in instances with large number of users became noticeably worse. It seems that it is not connected to poor avatar optimization in particular instance. After investigating, the framerate is roughly the same in instance with 30 people as in instance with 45+ people in previous patches.
1: Load into instance with at least 30 users
2: Notice framerate issue
3: If possible, compare the framerate with the same users and same amount of people without features added in aforementioned patch