HI,
congratulations ! you found the problem, the video with the "Hardware acceleration" option deselected has no problems and is smooth, the other option confirms the problem. I also tried saving on synology and apple 4ktv and not using hardware acceleration, works fine. I've never had this problem before, what do you think happened.... Os sequoia maybe? If necessary I could downgrade.
Anyway, thank you, now I can have fun again.