Hello!
I mostly record with 120 fps for an output of 30 fps.
I do this to be able to slow down the footage without making it stuttering.
I noticed that I cant just insert the 120 FPS footage and slow it down vie keyframes. It would stutter 
Am I doing anything wrong or is the way to pre-render the files to 30 fps with 4x length?
I guess your Project format is 1080p@30fps. Cinelerra interpret the frame rate of your clip to frame rate project (30fps), so when you change the speed to slow down, it jerks. There are different workarounds to solve the question.
This is a possible workaround :
http://www.visi1.org/cinelerra-gg_tutorials#SlowMotionWithHighFpsVideoSource
You can find the same video on Cin-GG YouTube Channel.
When you want to use the same clip in the timeline with 1x speed you have to use Reframe_RT effect to restore at the original freame rate.
very interesting and very useful!
Thank you very much!