怎么那么慢? /Zěnme nàme màn?/ 

怎么那么慢? /Zěnmeme màn?/ 

WHY (are you) SO SLOW? HOW COME (you are) SO SLOW?

The question word 怎么 /zěnme/ asks either HOW TO DO SOMETHING or HOW is it possible, that something happened or is so slow, expensive etc.

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Complete interactive list of question words >HERE<