Slacked up, meaning to slow down or decrease in intensity, can have a number of antonyms depending on the context in which it's used. If slacked up refers to a person's work output, possible antonyms include increased productivity, heightened efficiency, or boosted performance. If it relates to the pace of an activity, antonyms might include quickened pace, hastened progress, or accelerated momentum. In other contexts, antonyms for slacked up could be escalated effort, amplified intensity, or heightened commitment. The key to identifying appropriate antonyms for slacked up is to consider the opposite effect or result that one wishes to achieve.