迅

Bottom-and-left 辶 is the walking radical - the compressed form of 辵, picturing a foot in motion. Indexing radical, putting 迅 in the family of going-and-moving verbs (进 enter, 退 retreat, 迎 welcome, 追 chase). Carries the core meaning: 迅 names a speedy, swift motion, so the radical does the semantic work.

Inner 卂 supplies the sound - xùn straight through, no drift - and is itself an archaic pictograph of a bird in rapid flight, so the phonetic doubles as a perfect semantic match: swift bird-flight plus the walking radical = rapid travel. Same phonetic family: 讯 news, 汛 flood-tide, 训 train.