fast
侚 centers on speed and swiftness: being fast or quick. The other given meanings (showing, reciting, deep, using) are rare or archaic and do not form a clear conceptual cluster with 'fast'.
侚 combines the person radical 亻 with 旬, which likely provides the sound. The exact historical development is uncertain, and the character is rare.
A person 亻 beside 旬 (a ten-day period) suggests someone racing against a deadline: fast and swift.
For シュン, imagine a person running so fast they become a blur: 'shoon' — the sound of speed. Shoon -> シュン.