There are several synonyms that can be used instead of saying "very fast." Some of these synonyms include: rapid, speedy, swift, quick, accelerated, brisk, hasty, expeditious, fleet-footed, nimble, and lightning-quick. These words can make your writing more interesting and dynamic, as they add variety and nuance to your descriptions. However, it is important to choose the most appropriate synonym for the context you are writing in. For example, "accelerated" may be more suitable for a scientific text, while "nimble" may be better for describing a dancer's movements. Experiment with different synonyms and see which ones work best for your writing.