I extract the part 00:02:00.000 to 00:03:30.000 of a video with ffmpeg:

ffmpeg -i input.mp4 -ss 120 -t 90 -vcodec copy -acodec copy output.mp4

It works but the output video is jerky/choppy, i.e. not really smooth/fluid. Why?

The only log I have is:

[mp4 @ 000000000041c700] track 1: codec frame size is not set
[mp4 @ 000000000041c700] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
[mp4 @ 000000000041c700] pts has no value
     Last message repeated 7894 times

But when watching the original video and navigating to 00:02:00.000, the video is perfectly fluid/smooth. How to fix this?