Rapidly is an adverb used to describe something that happens quickly or at a fast pace. However, there are several other synonyms that can be used to convey the same meaning. Some of these synonyms include fast, swift, speedy, quick, hasty, accelerated, expeditious, and prompt. By using these synonyms in place of rapidly, you can add variety to your writing and make it more engaging for your readers. Choosing the right synonym depends on the context of your writing and the image that you want to create in the mind of your readers. Therefore, select a synonym that best fits your writing style and the tone of your piece.