Say a snake-person was between fifteen and eighteen feet long on average (this includes the "human" part of the body). That's around the length of an anaconda, but I imagine it would have to be thicker to support the human torso. Given that, how fast could a creature like this move?
Edit: Apparently this needs more details, so below is an example of the kind of creature I'm describing. The type of movement I'm asking about is similar to how a snake slithers, but with a human torso raised off the ground (similar to how snakes rear up). Hope that clears things up.
This creature could be considered a mythological creature called a Lamia.