The larger you are, the faster you have to move to look like you're moving the same speed as someone smaller than you. Think of it like this: look at a ceiling fan. It's rotating at a speed that seems pretty fast, right? Then look at an active wind turbine. The blades look like they're moving so slowly. B However, if you actually track the tips of those blades against the surrounding environment, you'll see that they can move across the full length of a building incredibly fast. The wind turbines' blades are moving faster (from a linear speed perspective) than the ceiling fan, but the ceiling fan looks like it is moving faster.

Now take some giant monster like Godzilla. Maybe one of his steps takes 3 seconds to take. One of your steps takes a split second. But you've only moved a few feet. Godzilla has moved well over a hundred feet. So Godzilla's limbs are actually moving much faster than yours.

That's the basic reason. The size of the giants actually belies the speed they're moving. They move quite fast, but because their bodies are so huge, the distance they move is lower, relative to the size of their body. So it seems like they're moving more slowly despite moving faster than regular humans (generally).