High speed is a term often used to describe fast-moving objects or processes. However, there are several synonyms for this term that can be used in different contexts. One such synonym is 'swift', which typically refers to something moving quickly and smoothly. 'Rapid' is another synonym, often used in situations where speed is crucial, such as in a medical emergency or during a disaster response. 'Quick' is another commonly used synonym, often used to describe the performance of a machine or the speed at which a task is completed. Overall, there are several alternatives to the term 'high speed', which can be used to add more variety and nuance to your writing.