rapidez
/rapiˈdeθ/ · B2 · noun · f
speed
⚡ rapidez = rapid + -ez, the noun ending that makes 'rapid' into 'speed'
Spanish rapidez comes from rápido (rapid) plus the suffix -ez, which turns adjectives into abstract nouns, like English -ness. So rapid-ez means rapid-ness, which is speed. The final z is pronounced like English th.