Speed in %, smooth or choppy, duration and frame rate.
Like an editing software: a clip shot at one frame rate, in a timeline at another, played at a speed in %. To be smooth, the screen needs a new frame every time the timeline shows one. Slowing down spreads the frames you shot: 120 fps at 20.8 % on a 25 fps timeline still gives 25 different frames a second, smooth. 25 fps at 40 % gives only 10: each frame is shown 2.5 times, the motion is choppy.
Editing software can invent the missing frames (frame blending, optical flow): it helps, with artefacts on fast or complex motion. Here, frames are simply repeated, like without that option.
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