I have seen some issues with builtin screen recordings in Gnome (Linux) (Shift - Ctrl -Alt -R) there creates .webm files, if you look at the properties of the recording, framerate is not specified.
shotcut and ffmpeg behaves stange on there files, it seams like it think they have 1000 fps and that make everything very slow.
To use there files i use ffmpeg to convert them to .mp4 and lock the frame rate to 30 fps.
ffmpeg -i infile.webm -filter:v fps=30 outfile.mp4
If you don’t add the fps=30, it takes forever and you end up with at .mp4 with 1000 fps