Hello Matti and thank you!
I checked the FCP manual and found nothing about "automatic frame rate".
Do you mean "Automatic Speed"? (http://lutslounge.com/retiming-using-automatic-speed-in-final-cut-pro-x/)
> 1. In the Final Cut Pro timeline, select the clip whose original frame rate you want to maintain.
> 2. Click the Retime pop-up menu below the viewer and choose Automatic Speed.
"preserve audio pitch is automatically selected" sounds helpful too. I will test it.
I noticed "Speed Transitions" is also checked by default. Which results does this give?
BTW I also read in the FCP manual about "optimizing":
> Create optimized media: This option transcodes video to the Apple ProRes 422 format, which provides
> better performance during editing, faster render times, and better color quality for compositing.
Nothing is mentioned here about a positive influence on deviant frame rates, so I'm afraid this wouldn't help in my case.