The word 快速 is formed through a straightforward semantic combination of its two constituent characters: 快 (kuài), which means "fast" or "quick", and 速 (sù), which means "speed" or "rapid". These two characters are near-synonyms, and when placed together, they reinforce and intensify the core concept of speed, creating a single, cohesive idea of "high-speed" or "rapid". This type of formation, where two meaning-related characters combine to clarify and strengthen a shared concept, is a common and intuitive method for building words in Chinese.