快速
kuàisù · HSK 3
fast
⚡🏃 快 (fast) + 速 (speed) = fast-speed, i.e. rapid
Compound of 快 kuài 'fast/quick' and 速 sù 'speed/velocity'. Both characters mean roughly the same thing, stacked for emphasis — a common pattern in Mandarin compounds meaning 'high-speed' or 'rapid'.
快 — heart radical 忄 + 夬 (phonetic)
快 is a semantic-phonetic compound. The heart radical 忄 carries the meaning, since the original sense referred to a feeling of pleasure or delight. The right side 夬 supplies the sound. The meaning later extended from 'pleased' to 'quick, fast', the dominant modern sense.