Antonyms for the phrase "are slothful" can be quite varied. For instance, you can use lively, active, diligent, or industrious to describe individuals who are the opposite of slothful. Other words that would suffice as antonyms include vigorous, tireless, bustling, productive, and efficient. By using these antonyms to describe an individual, you set them apart from being slothful and emphasize their work ethic and energy. Using antonyms can be a great way to communicate your message and convey your thoughts about an individual or group. Antonyms are useful because they provide a clear contrast and can help to create a more vivid and interesting piece of writing.