@Andrea
Thanks for your video. It explain very well the different cases.
If it don't waste your time and if you have time, you could transcode your source video (1080p) to mpeg and re-test in the same condition. So we (all) can see how i7-cpu "digests" the mpeg codec. I can not to do that because my pc is too much poor (usually I set Proxy scale to 1/4 for projects at 1080p). Yes, It still doesn't help to solve the problem.
Unfortunately I am not an expert in ffmpeg (like olaf or igor_ubuntu, and others), I just copy and paste after reading, but if you want to transcode your file with ffmpeg in the command line:
ffmpeg -i inputvideo.mp4 -c:v mpeg2video -q:v 2 -c:a copy outputvideo.mpg

@Phyllis
I saw your video. I learned another thing about Cinelerra, even if my old Notebook is really poor. Thanks!

@Terje
Thanks to you I can see that Mediainfo shows the GOP value. And I see that the mobile phones have all great GOP value to compress the file (my phone records with GOP=90). Thanks!

IgorBeg

PS:
Who has the Millennium falcon, please? ;-P