-ㄴ 듯이 means 'as if', and the phrase here is nearly always 미친 듯이 먹다, eating like a madman: 미친 듯이 먹었어 진짜 빨리 먹었어, 'you ate like a maniac, so fast'. Two lines swap in 홀린 듯이, 'as if under a spell', for eating you cannot stop yourself doing. Both are about volume and speed, and both are said with amusement rather than reproach.