Humans have reaction times on the order of 100-200ms. But interestingly if you measure reaction by you moving your hands or torso about 10%-20% of that time is just the time it takes for the signal to travel from your brain to the right muscle. Nerves aren't all that fast. That's why many reflexes happen outside the brain, closer to the relevant body part.

With that in mind, it would make a lot of sense if time perception scaled with body size. If you are a big elephant signals need to travel very far, and your muscles have to overcome a lot of inertia. If you can't react quickly anyways you don't need to perceive time as quickly. A fly has little inertia, fast signal times because of the short distances, fast processing because there just are fewer neurons, so there is more advantage to perceiving time more quickly