To be honest lunarians durability is odd, because some of their expressions clearly make it like seem they're in pain. I guess their skin in flame on doesn't get harder, the flame simply absorb the damage
My current theory is that Lunarian bodies, when in flame-on-mode, absorb force, and then allows them to release it.
When Zoro attacked King directly, and then King exploded, he was releasing the force he had taken.
When King increased his speed, he was releasing the force to move himself faster, but his flames must be off for this to happen. There are probably some limits we're not aware of, as well.
However, the Seraphim are cyborgs, clones made with both robotics and Lunarian DNA. They need an energy source, like Franky, to function. Since we haven't seen any of the Seraphim use King's super-speed, maybe they take the force they absorb, and use it to power themselves?