Record a time-lapse screen-capture directly in ffmpeg
This should work:
ffmpeg -framerate 1 -f x11grab -s 1920,1080 -i :0.0+0,0 -vf settb=\(1/30\),setpts=N/TB/30 -r 30 -vcodec libx264 -crf 0 -preset ultrafast -threads 0 out.mkv
EDIT: escaped parentheses from the shell