The opposite of "sped up" is "slowed down." When we slow down, we decrease our speed or rate of action, allowing us to become more deliberate and methodical in our approach. This can be useful in situations where we need to be more careful or make sure we don't make any mistakes. Slowing down can also help us conserve energy, allowing us to work more efficiently over a longer period of time. Other antonyms for "sped up" can include "decelerate," "delay," "procrastinate," "waste time," or "dawdle." Each of these words implies a decrease in speed or an increase in caution or sluggishness.