What surprised us was the prolonged delay we had to suffer during our departure from Heathrow.

prolonged 是动词 prolong 的过去分词形式，pro- 是前缀表示"向前"，long 表示"长"，合起来表示"延长、拖长"。prolonged 在这里作形容词用，意思是"持续很长时间的、延长的"。