Please sit down as quickly as possible.

as 在这里是连词，表示"像...一样"或"按照...的方式"。 quickly 是副词，意思是"快速地"。 possible 是形容词，意思是"可能的"。 as quickly as possible 是一个固定搭配，字面意思是"像尽可能快的那样"，实际表示"尽快"的意思。