Just curious if your Renoise behaves the same:
I m on Ubuntustudio 20.4 and I’m using Jack typically with 1024 samples. When I’m saving a larger project the GUI freezes and get a slight crackling sound when I hit Ctrl+S. No xruns appear just a slight crackle. I can image that saving a project increases CPU usage a lot. Or could it be due to the SDD. I thought that Renoise loads all samples into the RAM, so saving on the SDD should not interfere with the audio.