Usage Note
Rapidité is the abstract noun for speed as a quality or attribute; use vitesse when talking about a measurable rate (speed in km/h). Avec rapidité and avec vitesse are both used, but rapidité emphasises the quality of being quick rather than a numerical value. The adjective is rapide.
Examples
"La rapidité de sa réponse nous a surpris."
Natural Translation
The speed of his reply surprised us.

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