The phrase "slows up" denotes a decrease in speed, activity, or progress. Its antonyms include "speeds up," "accelerates," "expedites," and "hastens." Speeding up refers to the process of increasing activity or progress. It could also mean doing things quickly or efficiently. Accelerating suggests a more forceful increase in speed, while hastening implies urgency and the need to act with haste. Expediting focuses on making a process more efficient so that the outcome can be achieved more quickly. Therefore, if you want to express the opposite of slowing up, then use one of these antonyms, depending on the context and intensity of the action.