Speed doesn't scale which is the gist of it and why you also have to create slow motion in-camera and not by asking the actor in the Godzilla costume to "just move slowly" (explanation at the bottom). An easy intuitive way is to imagine you putting your arms out to your sides outstretched and then bringing your palms together as fast as you can and clap. It would take most folks less than a second with the average being something like 5-6 feet distance between the hands. Now imagine Godzilla scaled out to the point where one of his arms outstretched reaching the Sun and the other arm touching Earth. It takes roughly 8 minutes for the light from the sun to reach us. If he clapped his hands and it only took the same half a second it takes us, his hands would travel faster than the speed of light and go back in time.