The opposite of "lost speed" could be expressed in various antonyms. To regain speed, one could use verbs such as accelerate, quicken, hasten, or expedite. Alternatively, to maintain speed or keep a steady pace, words like maintain, continue, persevere, or endure could be used. In contrast, the antonyms of "lost speed" may suggest slowing down, such as decelerate, slow, or halt. These words imply a lessening of motion or a complete stop. Ultimately, the antonyms of "lost speed" depend on the context and what one aims to convey- whether it is acceleration, preservation, or deceleration.