低速

Used attributively before nouns: 低速行驶 (low-speed driving), 低速运转 (low-speed operation), 低速档 (low gear), 低速摄影 (slow-motion photography). Antonym: 高速 (gāosù, high-speed).

memory

低 (low) + 速 (speed) — fully transparent. Pair it with 高速 (high-speed) and 中速 (medium-speed) to remember the set.