quick, fast
寁 centers on speed: being quick or fast. The given meanings are straightforward and share this single idea.
寁 is a rare kanji. It combines 宀 (roof) with 疌, which likely contributes the sound and the idea of quickness. The exact historical development is uncertain.
Under a roof 宀, something moves with the speed of 疌 (a component associated with quickness). Imagine a fast-moving creature darting under a shelter.
For サン, picture a quick sunbeam darting under the roof: sun -> サン, and the light moves fast.