Why are larger animals so slow moving? Elephants, whales, even giants in shows like Game of Thrones. They all have slow, exaggerated movements. Would it be possible for a mythical creature to be extremely large but also quick (compared to humans)?
Metabolism is linked to body surface area to volume ratio. Smaller animals have a larger surface area to volume ratio. They lose heat quicker, they require more calories per kg of body weight, and their growth and reproductive cycles are faster.
Also, it takes physically more time to move a larger body. If a creature has a single step that moves 1 meter, it will take longer to move that limb than if the creature’s step is only 1cm.
By the same token, if you consider a heart beat, it takes longer for a heart to pump a large volume of blood from one chamber to the next than if it was a tiny heart.
It’d defy what we currently know about biology to have a massive creature that still had the speed and metabolism of, say, a hummingbird or a mouse.