One who is able to navigate a complicated, nuanced, or difficult situation in order to gain an advantage for themselves. A figurative use of the phrase's literal meaning, "one who is able to work quickly." Promoted to manager in less than half a year? My, he's a fast worker, isn't he? But the actress is a fast worker, deftly maneuvering through the more emotionally charged material to give her character an immediate presence in the film.