Flying by is an idiom that refers to something passing quickly or seeming to happen in a short amount of time. There are several synonyms for flying by, including zooming by, racing past, hurdling by, and speeding along. Other options include zipping by, flashing by, whizzing past, darting by, and sailing by. These synonyms can be used interchangeably with flying by, depending on the context and the tone of the sentence. No matter which option is chosen, they all convey the same sense of rapidity and movement.