Several parts combine into one character.

速 is a phonetic-semantic compound: 辶 (the "going / motion" radical) plus 束 (sù, "to bind / tied bundle" — providing both phonetic value and a hint of meaning). The composite implication is concrete: motion that is well-bound, tied tight, not loose or scattered — disciplined, efficient travel = fast. The 辶 radical anchors the entire vocabulary of motion and speed differentiation: 進 (advance), 迅 (rapid), 速 (fast), 遲 (slow / late), 達 (arrive), 通 (pass through). All these characters use the same "going" foot but specify different qualities of movement.

Korean reading "sok." Saturates the vocabulary of physics and transportation: 速度 (sokdo, speed / velocity), 迅速 (sinsok, rapid / swift), 高速 (gosok, high speed — central to Korean expressway names: 京釜高速道路 the Seoul-Busan Expressway), 急速 (geupsok, sudden / rapid), 音速 (eumsok, speed of sound — Mach 1), 光速 (gwangsok, speed of light). Korean technical and scientific writing relies heavily on 速.

Mandarin sù, 4th tone. 速度 (sùdù, speed), 快速 (kuàisù, fast / rapid), 高速公路 (gāosù gōnglù, expressway), 迅速 (xùnsù, swift). Note Mandarin's everyday word for "fast" in conversation is 快 (kuài, also meaning "happy" — homophonic flexibility); 速 is reserved for formal, technical, and compound contexts.

Japanese on-reading ソク (soku) — 速度 (sokudo, speed), 高速 (kōsoku, high speed), 音速 (onsoku, speed of sound), 迅速 (jinsoku, swift), 速報 (sokuhō, breaking news report — used constantly in news headlines). Kun-reading はやい (hayai, "fast") — 速い (hayai). Here Japanese makes a critical kanji distinction with the same kun sound: はやい spelled as 速い means "fast (in speed of motion)," while はやい spelled as 早い means "early (in time)." 電車が速い (densha ga hayai, "the train is fast") uses 速; 朝が早い (asa ga hayai, "morning comes early") uses 早. This 速い / 早い distinction is one of the most common kanji-disambiguation questions on Japanese language exams, and even native speakers occasionally swap them in handwritten text.

Memory aid: 辶 (going) plus 束 (tied / bundled) — efficient, undistracted motion = fast.