Everybody considers it impossible which he wants to finish the job in such a short time. that

what he wants to finish the job in such a short time.

在这里是个名词性从句,做为宾语补足语.

而 which 主要用来引导定语从句,这里显然不是.

通常which and that 是可以交替用的。 可是，which 也可以指“那个”。

the cat which belongs to Tim.
那只猫是Tim的