The character derives from its proto-form 悤, which represents a heart/mind (心) with a big dot on top indicating a hole. According to Qiu Xigui (裘錫圭) and Shen Pei (沈培), it indicates an open heart and an open mind. Its original meaning was "intelligent". Later, it became Phono-semantic compound (形聲 / 形声, OC *sʰloːŋ) : phonetic 囪(OC *sʰloːŋ) + semantic 心(“heart”), hence the dot on top became the pictogram of a window.
Eventually, the character evolved into 聰, i.e., Phono-semantic compound (形聲 / 形声, OC *sʰloːŋ) : semantic 耳(“ear”) + phonetic 悤(), in which the ear symbolizes somebody willing to listen and learn fastly through hearing.
莆田市荔城区档案馆[Putian City Licheng District Archives], editor (2022), “聪”, in 莆仙方言文读字汇 [Puxian Dialect Literary Reading Dictionary] (overall work in Mandarin and Puxian Min), page 39.