In Epic, here's leaf man mocking human moves in a slow manner:
Why do human movements look very slow from a tiny creature's point of view?
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Flies and other insects do indeed process visual information much more quickly than we do. Something about higher metabolism and a shorter optic nerve allowing information to be transmitted faster by orders of magnitude.
This is why, when you're trying to swat a fly, it always manages to escape at the last possible second. From its perspective, we do appear to be moving in slow-motion.
The movie is riffing off this detail, using the existence of an intelligent character who is also tiny to highlight the way tiny creatures would (and do) perceive us.
Slower movements can be used to exaggerate the size of objects. This comes from one of the 12 principles of animation, specifically timing. When done correctly, the amount of time things take can help convey different aspects of the item being animated. This video does an excellent job explaining animation in the game Shadow of the Colossus. The link starts around 2:56, which states:
You can use timing to communicate all kinds of stuff, like, for example, size.
Humans look slow partly because they are moving slowly relative to their body size. So, even if humans are actually much faster than a tiny creature in absolute terms, their movements will look slow.
In general, small creatures are able to cover a greater relative distance in small time. California mite covers 322 body lengths per second, which is equivalent to a person running roughly 580 m/s (ScienceDaily on this record)
Houseflies appear to be extremely quick when they fly at 2 m/s (8 km/h). Large ships don't look as fast at their top speed (> 50 km/h). Passenger airplanes move slowly across the sky at 500 knots (> 800 km/h).
Small creatures have much greater tolerance of acceleration, which is the rate of change of speed. This arises from Newton's law, force = mass times acceleration. The force to tear sinews or squash organs is much the same for a human and for a flea. But the mass of a flea is far smaller, and so the acceleration to which it can subject itself is far higher. This is also the reason that a squirrel can fall out of a tree and run away when it hits the ground, unharmed (or maybe a bit bruised). The same drop would likely kill you.
Small creatures may have faster perception. We can't know subjectively what it is like to be a sparrow, but we can prove that it must have control over its wings faster than a human is capable of, because otherwise it could not fly. Hummingbirds react and accelerate so quickly that the human eye can rarely follow them between a foot away from your hand and somewhere else.