Suppose you have at an old analog clock with a second hand that ticks. Look somewhere else in the room, then look at the clock. The first tick as you're looking at it takes much longer to occur, or at least that's the way it will seem to you.
The human eye doesn't move continuously, but moves in short rapid jumps. [0]. Your eye isn't perfect, and during those rapid jumps it produces blurry images. Rather than feeding those blurry images to the image recognition centers of the brain, those images are thrown out. That prevents your brain from using known-faulty data, but now you have a problem because motion tracking requires continuity. So you cheat. You take some of the images that arrived after the blurry images, extrapolate backwards to make an estimation of how things looked before, and then feed those to the motion tracking [1].
The neat thing is that this depends on how the second hand of your analog clock moves. If it ticks every second, then the brain's back-extrapolation to fill in the missing images sees a stationary second hand, and assumes that it was stationary in the past. If it rotates smoothly instead of ticking, then the brain's back-extrapolation sees the motion, assumes that the motion was constant during the past, and back-extrapolates accordingly. So if you glance over to a ticking clock, the first tick feels like it takes longer, but you don't have the same effect glancing over to a smooth motion.
[0] https://en.wikipedia.org/wiki/Saccade
[1] https://en.wikipedia.org/wiki/Saccadic_masking