The pre-designated speaking slots for each presenter were announced before the meeting started.

pre-designated 是由前缀 pre-（表示“预先”）和动词 designate（指定）的过去分词 designated 组成的复合形容词。pre- 表示“在...之前”，designated 表示“被指定的”，合起来意思是“预先指定的”或“事先安排好的”。