Room list: Loosely related performance investigations

[turt2live]

Things that people mention in the context of the room list but aren't directly related to the room list:

• room switch time is slow on a ~10 page roomlist due to forced layouts in _restoreSavedScrollState #14574 - Room switching time is slow [due to timeline]
• Performance regression on CIDER with new room list #14639 - CIDER is slow on room switch
• countRoomsWithNotif is super slow #14694 - Room notification counts are slow
• Room list filtering is painfully slow #14750