I use them, I think they help with deadening the sound. No,
They do not slow the bow down. They are just basically a string stop, the arrow is away before they actually do anything.
They weigh next to nothing and they do a great job of keeping things from getting under your bow string. I hunt from a tree stand a lot and hang my bow on a hook, they make it much easier and quieter to remove the bow from a hook.
Not cheap, someone is really making a killing selling them, but they work so I guess in the end that's all that matters.
RW