When you are feeling draggy — positively slug-like — you are sluggish, or slow. If cookie sales are sluggish at your bake sale, you are selling very few treats.
Just about anything can be sluggish. An old computer that takes forever to load a piece of software is sluggish, as is a car that can't quite make it up a big hill. And who isn't sluggish in the morning before the first cup of coffee?! Picture a slug moving its heavy little body across a basketball court... Still going? Yes. It will take that slug forever to get to the foul line at the sluggish pace it's moving.