Molasses is a gloppy, thick syrup made from sugar. If you are slow as molasses, then you should probably drop out of the track team.
The process of producing sugar from sugar cane involves several stages of boiling, and the syrup that results from this becomes molasses. The very darkest, thickest kind of molasses is called "blackstrap," which is more bitter than other molasses, but also contains all kinds of vitamins and minerals and is used by some people as a dietary supplement. The word molasses comes from the Latin mellaceus, "like honey."