The earliest siddur I own (London, 5569 [18808-09]) does not include Ketoret or Akeidah, despite purporting to follow the customs of the "German and Polish" communities [emphasis mine]. Since most of ...
I have seen in many siddurim, mainly in korbonos, the phrase "if you have time, add this parsha". How do you know if you have time? It seems obvious that if adding it would make you miss zman ...